Mitogen-induced lymphocyte stimulation responses in ambulatory patients with major depressive disorder did not differ from those of matched controls. Lymphocyte responses in hospitalized schizophrenic patients and in patients hospitalized for elective herniorrhaphy similarly did not differ from responses of controls. The number of peripheral-blood T cells was decreased among the ambulatory depressed patients but not in the schizophrenic patients. These findings, together with previously reported decreased lymphocyte function in hospitalized depressed patients, suggest that decreased lymphocyte function is associated specifically with depression and not related to hospital effects or nonspecifically to other psychiatric disorders. The results also suggest that altered immunity in depression may be related to severity of depressive symptoms.