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. METHOD: 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.