One of the most consistently reported findings in depression has been leucocytosis. In 33 in-patients with major depressive disorder, we assessed white blood cell count (WBC) and lymphocyte subsets four times over a period of 6 weeks. The control group consisted of 44 healthy subjects. Shortly after admission, we detected significant increases in the numbers of leucocytes, granulocytes, platelets and monocytes in the depressed patients. Patients who recovered well during hospitalization showed a decrease in monocyte counts, whereas those with slower clinical improvement had significantly higher monocyte counts than the control group. This longitudinal study demonstrates that monocytes may play a role in the acute phase of depression and could provide an explanation for immunological dysfunction in depressive states.