Twenty-six children and adolescents with severe primary obsessive-compulsive disorder receiving long-term clomipramine hydrochloride maintenance treatment (mean +/- SD, 17.1 +/- 8.3 months; range, 4 to 32 months) entered an 8-month double-blind desipramine hydrochloride substitution study to assess the necessity of continued drug treatment. All patients received clomipramine for the first 3 months, then half continued with clomipramine therapy (nonsubstituted group) and half had desipramine blindly substituted for the next 2 months; all subjects again received clomipramine for the last 3 study months. Eight (89%) of nine of the substituted and only two (18%) of 11 of the nonsubstituted group subjects relapsed during the 2-month comparison period. Long-term clomipramine treatment seems necessary for this population of children and adolescents with obsessive-compulsive disorder. However, even patients receiving maintenance clomipramine treatment throughout the entire study had continued obsessive-compulsive symptoms, which varied in severity over time.