A preponderance of patients with obsessive-compulsive disorder (OCD) experience little or no improvement in their symptoms when treated with serotonin reuptake inhibitors (SRIs). It is hypothesized that SRI-refractory patients may have altered serotonin neurotransmission different from patients responsive to SRIs, or that they may have abnormalities in their dopamine function. When drugs affecting serotonin function (e.g., tryptophan, fenfluramine, lithium, buspirone) are added to SRI therapy in SRI-refractory patients, results are mixed and not consistently encouraging. However, when drugs affecting dopamine function (e.g., pimozide, haloperidol, risperidone) are added to SRI therapy in SRI-refractory OCD patients, individuals with either a personal history or family history of tics experience a reduction in their symptoms.