In a year when most state legislatures are backing large tuition increases to close budget shortfalls, lawmakers in Illinois have agreed to guarantee that tuition won't rise during a student's four years at a public college in the state.

The "truth in tuition" bill was signed by Gov. Rod R. Blagojevich on Tuesday at the University of Illinois at Urbana-Champaign. The law, which takes effect in the 2004-5 academic year, locks in tuition rates for students for four consecutive years, or for five years in those majors that require extra courses.

"This legislation will bring stability to the cost of a college education at our public universities," Mr. Blagojevich said in signing the bill. "The tuition a student pays as an incoming freshman should be the same tuition the student and his family pay as a senior."

The Democratic governor pushed for the tuition freeze even as he cut higher-education spending by some $112-million this year in order to deal with a $5-billion budget deficit. Combined with the spending cuts, university officials say the tuition freeze will in some ways have a contradictory effect, by forcing them to raise rates substantially next year in order to bring in enough new revenue.

"The thinking is that there will be perhaps larger-than-normal increases next year," said Don Sevener, a spokesman for the Illinois Board of Higher Education. "The institutions will need to decide at what level they need to set tuition in order to sustain revenue."

While some small private colleges have had tuition guarantees in place for years -- and Western Illinois University has had one since 1999 -- the new Illinois law marks the first time that a state will force all public colleges to lock in their rates. And there are good reasons that no other state has adopted such a policy, said Travis Reindl, director of state policy analysis at the American Association of State Colleges and Universities.

"Unlike a private college, the revenue streams for public colleges are less predictable because they depend on the legislature," he said. "This puts all kinds of pressure on a university. It's darn good PR, but it's not very good policy."