OBJECTIVE This study was to investigate the relationship between serum prolactin and pregnancy-induced hypertension. METHODS Maternal serum prolactin was estimated by radioimmunoassay in 89 normal pregnant women and 52 patients with severe pregnancy-induced hypertension at 28 to 42 week gestation. RESULTS In normal pregnancy, prolactin level increased progressively from a mean value of 270 mg.L-1 in the 24th week to 440 mg.L-1 in the 34th week and decreased progressively from the 38th week to the 40th week. In severe pregnancy-induced hypertension increased progressively too; 31 of 52 patients with severe prolactin showed high prolactin levels in zone A (> mean value + standard of prolactin values in the normal pregnancy), 10 patients in zone B (mean + standard to mean) and 11 patients in zone C (< mean). CONCLUSIONS These results suggest that high prolactin levels may play an important role in the pathogenesis of severe pregnancy-induced hypertension.