Our purpose was to describe the incidence and effects of serious obstetric complications on maternal outcome in pregnancies complicated by HELLP syndrome. A prospective cohort study was performed on 442 pregnancies with HELLP syndrome managed at this center from August 1977 through July 1992. Of 437 women who had 442 pregnancies with HELLP syndrome; 309 (70%) of the cases occurred ante partum and 133 (30%) post partum; 149 (11%) developed at < 27 weeks and 80 (18%) at term. Maternal mortality was 1.1% (five patients). Serious maternal morbidity included disseminated intravascular coagulation (21%), abruptio placentae (16%), acute renal failure (7.7%), pulmonary edema (6%), subcapsular liver hematoma (0.9%), and retinal detachment (0.9%). Fifty-five percent of patients required transfusions with blood or blood products, and 2% required laparotomies for major intraabdominal bleeding. Abruptio placentae was strongly correlated with the development of disseminated intravascular coagulation (p < 0.0001), acute renal failure (p < 0.001), and pulmonary edema (p < 0.01). Moreover, there was a strong association between pulmonary edema and acute renal failure (p < 0.0001). There were no differences in laboratory findings between HELLP syndrome before and after delivery; however, women with postpartum HELLP syndrome had significantly higher incidences of pulmonary edema and renal failure. HELLP syndrome is associated with serious maternal morbidity, especially when it arises in the postpartum period.