Two- and 4-week double-blind placebo-controlled trials of lithium augmentation of ongoing fluvoxamine treatment trials were conducted in 20 and 10 patients, respectively, with primary obsessive-compulsive disorder (OCD) who had failed to respond to fluvoxamine alone. Although 2 weeks of double-blind lithium augmentation produced a small but statistically significant reduction in obsessive-compulsive symptoms, most patients did not have a clinically meaningful response. Furthermore, there was no statistical or clinical improvement in obsessive-compulsive symptoms during the subsequent 4-week double-blind, placebo-controlled trial of lithium augmentation. On the basis of treatment response criteria, only 18% and 0% of the patients responded to lithium augmentation of fluvoxamine during the 2- and 4-week treatment trials, respectively. In light of the previously reported 44% response rate to lithium augmentation in treatment-resistant depressed patients on fluvoxamine, the results of this study suggest that pathophysiological differences may exist between OCD and depression. The routine use of lithium augmentation in the management of patients with OCD who are refractory to serotonin reuptake inhibitors is not supported by these findings.