In a sample of 40 youths (ages 11-17) with obsessive-compulsive disorder (OCD) and mood disorders who were treated with behavior therapy, 20 patients received serotonin reuptake inhibitors (SRIs) and 20 did not. In open-label clinical treatment, 30% of the patients (6/20) treated with SRIs developed manic or hypomanic symptoms (5/15 on fluoxetine, 1/1 on sertraline). Symptoms included impulsivity, grandiosity, pressured speech, and disinhibition and did not resemble akathisia or "behavioral activation." These behaviors emerged despite gradual dose elevation (2-5 mg/wk), conservative dosing (maximum 40 mg daily), and careful weekly outpatient monitoring of each patient. Fluoxetine-induced mania occurred at doses as low as 10 mg daily. It is unclear whether mania/hypomania would appear in OCD children without comorbid mood disorders or, alternatively, whether OCD is a stronger risk factor than mood disorder for manic switch in SRI-treated youths. Clinicians are advised to be aware of the risk and to be vigilant in monitoring manic and hypomanic behaviors when using SRIs to treat OCD in youth, even with low doses and gradual dose elevation.