OBJECTIVE Few studies, with conflicting results, have examined the occurrence of thrombocytopenia with valproate administration. This study assessed whether the platelet-lowering effects of valproate are increased among elderly psychiatric patients. METHODS The charts of 39 psychiatric inpatients taking valproate were reviewed. Information on hematologic function and on doses and serum levels of valproate at sequential intervals over a 12-month period was abstracted. Data for subjects age <60 years (nonelderly subjects) and those age >/=60 years (elderly subjects) were compared. RESULTS More than half (53.8%) of the elderly patients but only 13.0% of the nonelderly patients had at least one episode of thrombocytopenia. This difference corresponded to a significantly greater lowering of platelet counts among elderly patients than among nonelderly patients. CONCLUSIONS The high rate of thrombocytopenia in our study suggests the need for routine monitoring of platelet counts among patients taking valproate, particularly among elderly patients.