Prospective contemporaneous data on the outcome of pregnancies in women with chronic hypertension are sparse. Indices of maternal and perinatal morbidity and mortality were determined in 822 women with chronic hypertension with data prospectively collected and rigorously validated. The incidence of superimposed preeclampsia was 22% (n=180) with early-onset preeclampsia (<or=34 weeks gestation) accounting for nearly half of these cases. Delivering an infant <10th customized birthweight centile complicated 48% (87/180) of those with superimposed preeclampsia and 21% (137/642) in those without (relative risk [RR] 2.30; 95% confidence intervals [CI] 1.85 to 2.84). Delivery at <37 weeks gestation occurred in 51% of those with superimposed preeclampsia (98% of these iatrogenic) and 15% without (66% iatrogenic) (RR 3.52; 95% CI 2.79 to 4.45). Using multiple logistic regression, black ethnic origin, raised body mass index, present smoking, booking systolic blood pressure of 130 to 139 mm Hg, and diastolic blood pressure of 80 to 89 mm Hg, a previous history of preeclampsia or eclampsia and chronic renal disease were identified as risk factors for superimposed preeclampsia. Adverse maternal and perinatal outcomes occur in women with chronic hypertension; the prevalence of infants born small for gestational age and preterm is considerably higher than background rates, and is increased further in women with superimposed preeclampsia. Use of customized birthweight centiles provides more accurate determination of fetal growth restriction and highlights the need for greater fetal surveillance in these women. Paradoxically, smoking is an independent risk factor for superimposed preeclampsia in chronic hypertension, in contrast to the protective effect in low-risk pregnant women.