The platelet count in 550 patients with gestational hypertension was significantly lower and the mean platelet volume significantly higher than in normal pregnant women. Both the platelet count and volume became increasingly abnormal when hypertension was accompanied by oedema, proteinuria or both, and women with severe pre-eclampsia or eclampsia had the lowest platelet counts and the highest mean platelet volume. The proportion of patients with thrombocytopenia and/or macrothrombocytosis also varied with the severity of the clinical presentation. Fibrinogen degradation products were found mainly in fully developed pre-eclampsia. These findings confirm the concept of a rapid platelet turnover caused by low-grade disseminated intravascular coagulation in gestational hypertension. The platelet pattern in essential hypertension is similar to that seen in normal pregnancy.