CHICAGO (AP) _ Once touted as Chicago's Taj Mahal, the space-age State of Illinois Center has become the focus of lawsuits over its design and occupants' complaints about heat, acoustics and just about everything else.

The $172 million state office building - the state's costliest - has sparked controversy ever since its completion in 1985.

The circular, 17-story, red-and-blue glass building designed by renowned architect Helmut Jahn has been called a blob and an upside-down cupcake. Critics also say it's noisy and lacks ventilation.

Admirers call it a landmark and an architectural masterpiece. Gov. James R. Thompson, whose Chicago offices are housed on the 16th floor, has called it ''the first office building of the year 2000.''

The governor, however, is withholding comment now that the state attorney general's office has filed a $20 million lawsuit accusing the building's architects and engineers of professional malpractice.

The lawsuit, filed earlier this month, alleges that summer temperatures have exceeded 110 degrees in some parts of the center, where 3,000 state employees work.

Jahn, of Murphy-Jahn architects, has filed his own lawsuit, alleging that the problems are due to faulty mechanics, not the center's design.

The building, dedicated in 1985, covers a city block. It is covered with 24,600 shimmering glass panels, rising from a pink and white base.

''It has incredible shock value,'' said Thomas Beeby, director of the school of architecture at the University of Illinois-Chicago. ''It's such a powerful image, you either love it or hate it.''

Robert Irving, a professor of humanities from the Illinois Institute of Technology, once likened its shape to that of a ''fat alderman.''

The air-conditioning system is being fixed and should be working by summer, said Gary Skoien, director of the state Capital Development Board, which oversaw construction of the center.

The innovative air-conditioning system circulates air over ice produced at night when energy costs are lower.

Problems with the air-conditioning ''certainly hindered the acceptance of the building,'' Skoien said.

But he adds: ''I think the building itself is very successful. It's a tremendous tourist attraction ... The air conditioning thing just allowed some people who had something against the building to say, 'I told you so.'''

Jim Goettsch, executive vice president of Murphy-Jahn, says the controversy ''has generated more business than it's lost.''

''I can't really point to any jobs that we've lost because of that building,'' he said. ''A lot have come in because people liked the building.''

Richard Whitaker, dean of the College of Architecture, Art and Urban Planning at the University of Illinois' Chicago campus, thinks the building will weather the storm.

''Typically the kinds of problems that seem to exist now in the building are the kinds of things that people won't remember'' years from now, he said.

The state's lawsuit does not address complaints about aesthetics, said First Attorney General Jill Wine-Banks.

''Ugly buildings are not illegal,'' she said.