I will go further. If this is what the whole modernist project of the 20th century was leading up to, then it has been worth the wait. All Frank Gehry buildings are interesting, some are very good indeed, but with this one, at 68, he has achieved greatness. There are more buildings of an extreme nature to come from him, but I will be surprised if he manages to surpass this.

It is "just" an art gallery, the result of the Basque capital of Bilbao buying itself international status twice over: first by signing up New York's Guggenheim for the project, paying the entire $100m cost of the building and almost half its running costs, then by letting Gehry design it. Gehry has been living this building since he won the competition in 1991, and refining it since construction began four years ago. Guggenheim and Gehry are, in the art world, blue-chip brand names. Bilbao has others: it has Norman Foster for its Metro, Michael Wilford for its new transport interchange, it has the now mandatory Santiago Calatrava bridge, and so on: but the Guggenheim is the building that best defines Bilbao's $1.5 billion aspirations for the year 2000.

The result of the collaboration between Gehry and Thomas Krens, the Guggenheim's director, is a museum (Americans do not make our distinction between "museum" and "gallery") that is distinctly but not defiantly fin de siècle. I came expecting decadence but ­ with the exception of Jeff Koons's giant floral puppy in the forecourt ­ I did not find it. It was this puppy that blundering Eta terrorists tried to use as a mortar launch site earlier in the week, killing a policeman in the process.

This is a big building for big art. It is as simple as that, and as symbiotic as that. The steady growth in scale of contemporary art works has led to a need for industrial-sized spaces to house them. Contrariwise, artists immediately respond to the space provided. So in Bilbao, Gehry has given Krens one room that is the biggest single gallery space in the world ­ 450ft long, 80ft wide and lofty. Immediately, the sculptor Richard Serra, an old friend of Gehry's, like many of the American artists on display, was commissioned to produce a permament piece for the space. Snake, the sinuous rolled-steel work that Serra created, responds directly to the swooping forms of the building. The paradox is that, mighty though the Serra is, it is almost dwarfed by the space. Gehry jokes that he now expects the fashionmongers of the art scene to move into thimble-sized miniatures. This is unlikely.