ABSTRACT A 13-year-old boy with attention-deficit hyperactivity disorder and conduct disorder developed manic symptoms during a trial of fluoxetine. A prior methylphenidate trial had failed, and induced irritable and dysphoric effects at 0.5 mg/kg daily. In view of the mood symptoms, a trial of tricyclic antidepressants was recommended, but the family rejected the recommendation and preferred a trial of fluoxetine. Fluoxetine was initiated at 5 mg daily and increased at weekly intervals by 5 mg. The symptoms of ADHD were not improving, but no adverse effects were attributed to the medication. After 2 days on fluoxetine 20 mg, clear-cut symptoms of mania emerged. The manic symptoms did not subside after discontinuation of fluoxetine, but were controlled by lithium and carbamazepine. The manic symptoms returned when these medications were discontinued by the family, and were again controlled when lithium was reinstituted. The episode fulfilled DSM-III-R criteria for nonpsychotic mania. By history, this patient had not experienced a depressive, hypomanic, or manic episode prior to treatment with fluoxetine. Retrospectively, it appears that the patient began to show evidence of hypomania or mania during the second week of fluoxetine treatment while on 10 mg daily. It is recommended that clinicians be alert to the possibility of a manic switch appearing during fluoxetine treatment, even at low doses of medication.