A new long-term study finds the majority of people diagnosed with attention-deficit hyperactivity disorder in childhood continue to have ADHD or another psychiatric disorder in adulthood, suggesting that the condition is a chronic disorder with potentially serious outcomes.

The research, to be published Monday in the journal Pediatrics, identified 362 children with ADHD from the group of all children born in Rochester, Minn., between 1976 and 1982 and followed them into adulthood, for about 20 years, comparing them with 335...