It is estimated that 15 to 30 percent of individuals with bipolar illness have an episode of depression or mania before the age of 20 years. For this study, bipolar I illness was defined using the criteria in the Diagnostic and Statistical Manual of Mental Disorders, third edition (DSM-III); these criteria include the presence of manic disorder without psychosis. Thirty-seven adolescents with bipolar I illness were evaluated after their symptoms had been stabilized with lithium, the drug of choice in treating this disease. After the patients were released from the hospital, they were monitored every four to six weeks for 18 months. The frequency of depressive and manic relapses was observed in the subjects during this follow-up period. Of the total group, 24 patients (65 percent) continued lithium maintenance therapy throughout the follow-up period without interruption, and 13 patients (35 percent) discontinued their therapy against medical advise. There were no significant differences between these two groups with regard to sex, age, or medical history. More than half (57 percent) of the total group relapsed during follow-up, but the relapse rate among those who discontinued lithium was three times that of compliant patients. Twelve out of the 13 noncompliant patients relapsed, while only nine out of the 24 patients who continued to take their medication relapsed. Some of the clinical and theoretical implications of these findings are discussed. The relapse rate in these adolescents was similar to that which has been observed in comparable adult groups. In general, patients who experience early relapse run a higher risk of subsequent relapse. (Consumer Summary produced by Reliance Medical Information, Inc.)