OBJECTIVE: Our purpose was to describe the incidence and effects of serious obstetric complications on maternal outcome in pregnancies complicated by HELLP syndrome. STUDY DESIGN: A prospective cohort study was performed on 442 pregnancies with HELLP syndrome managed at this center from August 1977 through July 1992. RESULTS: 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. CONCLUSION: HELLP syndrome is associated with serious maternal morbidity, especially when it arises in the postpartum period.