Scanning electron microscopy was used to study the morphologic characteristics of erythrocytes in women with preeclampsia and eclampsia. In nine nulliparous women with eclampsia, the proportion of abnormal red cells (schistocytes, echinocytes, and spherocytes) was significantly greater than in 12 normally pregnant control women (p less than 0.001). Likewise, the proportion of these abnormal erythrocyte forms was significantly greater in 12 nulliparous women with preeclampsia than in 25 normally pregnant control women (p = 0.009). Six women with mild pregnancy-induced hypertension without proteinuria were studied before and after magnesium sulfate therapy had been given, and no differences in the proportion of abnormal erythrocyte forms were found. Evidence for microangiopathic hemolysis, manifest by reticulocytosis and thrombocytopenia, was apparent with eclampsia, although only reticulocytosis was identified in women with preeclampsia. There was evidence for hepatic dysfunction in more than half of the women with eclampsia, and in nearly one third of those with preeclampsia. We speculate that compositional changes in the membrane, induced by plasma/erythrocyte lipid interchanges, which are predisposed by normal pregnancy and amplified by severe preeclampsia, especially in the presence of liver dysfunction, may have participated in the genesis of the red cell abnormalities observed. Furthermore, these abnormalities in the cell membrane may increase erythrocyte susceptibility to microangiopathic hemolysis.