Haemorrhagic tendencies, thrombocytopenia and/or platelet dysfunction have been reported in patients under sodium valproate (VPA) treatment for epilepsy. The true incidence and significance of these side-effects are discussed in the present paper based on case reports and systematic enquiries. The following conclusions emerge: VPA is responsible for thrombocytopenia and platelet dysfunction. These side-effects are dose-related, and the daily dosage should never exceed 40 mg/kg. Their mechanism is obscure and several explanations have been put forward. The effects of VPA on blood platelets may be considered as idiosyncratic in nature and have little clinical relevance unless patients contract an infectious disease or require surgery; only in such cases would a coagulation study be required.