The risk is greatest for those born following a pregnancy lasting less than 32 weeks.
Compared with normal-term babies, they are three times more likely to be hospitalised for a psychiatric problem at age 16 or older, researchers found.
Very premature babies have more than twice the normal chance of developing forms of psychosis. Their risk of bipolar disorder is increased more than sevenfold, while the chances of developing eating disorders are raised 3.5 times. Experts stressed that the chances of a premature baby having a serious psychiatric problem were still small.
The new research, reported in the Archives of General Psychiatry, is based on a study of almost 1.5 million Swedish records from 1973 and 1985.