Maternal smoking reduces the risk of pre-eclampsia, but has been reported to increase the risk of adverse outcomes related to the disease. We used data from the trial of Calcium for Pre-eclampsia Prevention (CPEP) to explore whether clinical manifestations of pre-eclampsia were altered by maternal smoking. CPEP was a randomised study of 4589 nulliparous women conducted in five US medical centres. Smoking history was obtained at study enrolment and women were monitored for the development of hypertension, proteinuria, and other medical complications. Among pre-eclamptic women (n=274), the risk of severe disease was not elevated in smokers (adjusted odds ratio 0.87 [95% confidence interval (CI) 0.30, 2.51]). Compared with non-smokers, gestational age (days, +/-SE) at onset of pre-eclampsia was not reduced in smokers (264.8 +/- 1.5, and 268.2 +/- 5.5, respectively, P=0.48). The smoking-attributable deficit in birthweight was not increased in pre-eclamptic women compared with normotensive women (97 g [95% CI -49, 244] and 185 g [95% CI 141, 229] respectively). In conclusion, among women who developed pre-eclampsia, smoking during pregnancy was not associated with disease severity. We found no evidence that pre-eclampsia and smoking act synergistically to restrict fetal growth.