The concentration of cytokines such as Interleukin-6 (IL-6) has been reported to be elevated in depressed and schizophrenic patients and, in healthy persons, upon stress. Interleukin-6 plasma levels were determined in depressed (n = 12) and schizophrenic (n = 32) patients during the acute state of illness and after remission at approximately 8 weeks after admission and were compared with healthy controls (n = 12). Patients were diagnosed according to DSM-III-R by the Structured Clinical Interview (SCID). Severity of illness was assessed for depression by the Montgomery Asberg Depression Rating Scale (MADRS) and for schizophrenia by the Brief Psychiatric Rating Scale (BPRS). Interleukin-6 plasma concentrations were elevated during the acute state either of depression or of schizophrenia if compared to controls. After remission, IL-6 concentrations in depressed and in schizophrenic patients had decreased and did not differ significantly from controls. We hypothesize that the elevated IL-6 levels during the acute state of depression or schizophrenia may reflect an unspecific stress response.