Background. Chronic hypertension during pregnancy is associated with an increased risk for birth of small for gestational age offspring. The aim of this study was to determine whether the risk remains after consideration of maternal characteristics and superimposed pre-eclampsia. Method. A population-based cohort study based on the Swedish Medical Birth Register 1992-98 and comprising 560,188 women aged 15-44 years with singleton pregnancies including 2,754 women with chronic hypertension. The register contains data of maternal characteristics besides maternal and fetal complications. Multiple logistic regression analysis was used. Small for gestational age was defined as birth weight less than -2SD adjusted for gestational age and sex. Results. Chronic hypertensive women have an independent risk for small for gestational age (OR 3.1; 95% CI 2.7-3.7) when controlling for confounding of maternal characteristics such as age, parity, BMI, smoking, and ethnical origin. After introducing superimposed pre-eclampsia in the model the risk remains but decreases (OR 2.4; 95% CI 2.1-2.9). Conclusion. Chronic hypertension is an independent risk factor for birth of small for gestational age offspring. Pre-eclampsia is a strong mediating factor.