OBJECTIVE To investigate whether the increased membrane fluidity postulated as a possible contributing factor to the hypertensive states of pregnancy is related to the lipid composition of the erythrocyte membrane. METHODS An observational case control study. METHODS 30 women with pregnancy induced hypertension, 26 normotensive pregnant women matched for gestational age, and 10 normotensive non pregnant nulliparous women. METHODS Erythrocyte membranes were prepared from venous blood samples obtained from all the women. METHODS Lipid analysis, including cholesterol to phospholipids ratio, distribution of phospholipid classes and fatty acid composition of total phospholipids in erythrocyte ghosts. RESULTS The cholesterol/phospholipid ratio was significantly higher in the women with pregnancy induced hypertension compared with the normotensive pregnant women (mean 1.24, SD 0.31, 95% CI 1.12 to 1.35 vs mean 0.90, SD 0.09, 95% CI 0.86 to 0.94; P less than 0.01). Normotensive non-pregnant erythrocyte membrane cholesterol/phospholipid ratio was 0.88 (SD 0.11, 95% CI 0.79 to 0.96). The percentage distribution of different phospholipid classes and fatty acid composition was similar in all the four groups. CONCLUSIONS The increased cholesterol/phospholipid ratio of the erythrocyte membrane found in pregnancy-induced hypertension represents one factor involved in the pathophysiology of this condition and a possible biochemical marker of the disease.