Reduced fetal growth in babies born preterm may be associated with reduced later blood pressure, but in children born at term, higher blood pressure. It was hypothesised, therefore, that maternal smoking in pregnancy, associated with reduced fetal growth, programmes later blood pressure differentially according to length of gestation. Six hundred and eighteen children born preterm and now aged 7.5 to 8 years were studied prospectively. Systolic blood pressure in children from smoking compared with non-smoking mothers was significantly lower in those born before 33 weeks' gestation and significantly higher in those born at 33 or more weeks. Within the range 0-40 cigarettes per day until delivery (after adjusting for potentially confounding factors, including social class and current weight) each 10 was associated with a 1.5 mm Hg fall and 2.9 mm Hg rise in pressure for children born below or above 33 weeks' gestation respectively. Similar though smaller differences were seen in diastolic pressure. These data support our hypothesis that later effects of insults impairing fetal growth are gestation dependent, and provide the first evidence that maternal smoking may have long term consequences for blood pressure in children.