JUST before Andy Warhol’s death there was an exhibition in a London gallery of his latest self-portraits, done in the previous six months. They were all based on the same photograph, with the famous platinum wig in wild disarray, transferred to canvas and each suffused with a different pastel shade or, in some cases, a pale military camouflage. As I was looking round, one of the dealer’s minions descended on me.

“Isn’t it wonderful?” he cried. “I think these are probably the most important and moving paintings of the 20th century!”

“Um, well . . ,” I replied.

Now we all have the chance to look at them anew. They bring up the rear of the Scottish National Gallery of Modern Art’s new, comprehensive exhibition of Warhol’s work in self-portraiture throughout his career. And very extensive it proves to be.

Of course, virtually everything Warhol did, he did on an industrial scale: not for nothing was his studio called the Factory. But even so, the assembly of more than 80 self-portraits comes as a surprise.

They begin in 1948, when Warhol was just 20, and end in 1986, when he was 48. The earliest works are drawings, in the style that has become more generally familiar in the past few years through the endless production of gift booklets re-using his advertisement art on the subject of shoes, food, angels and such. Before this recent spate, the drawings were produced chiefly to show that the artist was perfectly capable in conventional forms beforehe turned exclusively to photographs as the basis of his art.

The Warhol drawings have a touch of sophisticated naivety curiously reminiscent of Lucian Freud. And much as Freud’s teenage work makes one wonder if he can have known the art of Edward Burra, so Warhol’s first surviving works suggest a close acquaintance with New York artists of the Fifties who juggled fine art with commercial, notably Ben Shahn and Saul Steinberg. But whatever his models, if any, such Warhol drawings as the Fifties self-portrait with his hands covering his eyes already have extraordinary maturity and confidence in their gift to be simple.

Perhaps it all came to him too simply. Or maybe he realised that this style was too closely associated with his advertising work to let him be taken seriously as a fine artist. Possibly he simply wanted to do something more controversial and newsworthy.

But whatever the reason for the change of direction, it did change radically around 1962, the date of the first Campbell’s soup tins and the first photographic screen-prints. His first subjects for this treatment included Marilyn Monroe, Elvis Presley and Coca-Cola bottles, as well as the first death-related images of road accidents. By the following year he started to apply the same “impersonal” procedures to his own image.

The first self-portrait painting in this show at once hits on the distinctive Warhol look: a photo-booth picture is blown up, printed on canvas and tinted light blue in one version, turquoise in another — with the clear implication that it could be mechanically reproduced ad infinitum, or at least until the silk screen wore out.

And where is Warhol himself in the process? He did not take the original photograph, he did not make the silkscreen (the strip of photographs has written instructions to someone else to do that) and, very likely, he did not even do the transfer to canvas himself. In fact, by traditional standards his intervention was more that of an art director than an artist proper. But for him that was the whole point: in our present, machine-ruled world, anyone could make a Warhol.

Later, when he was suing two of his Factory assistants, Gerald Malanga and Bridget Polk, for taking him literally and making their own, I asked him whether this was not a little inconsistent with his stated principles. “Ah,” he said, “that was art, this is money.”

And there seems little doubt that his principles on the role of the mechanical and the impersonal in modern art were quite sincerely held. Why else would he have virtually deserted his proved and prize-winning skills as a draughtsman in order to take up a technique which he himself insisted was hardly a skill at all?

A few years ago there was a widely seen museum exhibition called The Warhol Style, and everyone knew exactly what that meant. Undeniably, by adopting a style which seemed to detach the work decisively from the maker, he had created something intensely personal — as well as, ironically, transforming the face of 20th-century art. Having invented the style, and in the process virtually invented the Pop Art aesthetic, Warhol continued to exploit his new discovery and defend his rights to it against a host of imitators.

The big question that the Edinburgh show should help to answer is: what about Warhol himself? He was tremendously successful in terms of money, fame, reputation, and, in spite of being seriously injured when shot by a crazed disciple, he continued to play a prominent role on the New York, and therefore the world, art scene. But did he develop at all? Are those last, Hallowe’en-type portraits really among the most important and moving 20th-century paintings?

Probably not. The latest works differ hardly at all in technique or even “quality of vision” from the first of their kind, more than 20 years earlier. Well, Warhol would not be the first artist to plant his roots early, then rest on his laurels. But then, if Tracey Emin is right in saying that the mark of a great artist is that he or she produces at least one work which alters the course of art history, then who can deny Warhol the accolade?

• Andy Warhol: Self-Portraits is at the Scottish National Gallery of Modern Art, 75 Belford Road, Edinburgh (0131-624 6200), to May 2