OBJECTIVE Smoking has been suggested to reduce the risk of pregnancy-induced hypertension (PIH). We have analyzed the association between smoking and risk of PIH using data from a case-control study conducted in Italy. METHODS Cases were 215 women who gave birth on randomly selected days at a network of obstetric departments and with a diagnosis of PIH, i.e. diastolic pressure above 90 mmHg on at least two occasions 24h apart. Controls were 1222 women (median age 30 years) who delivered at term healthy infants on randomly days at the same hospital where the cases had been identified. RESULTS In comparison with never smokers, current smokers at conception were at decreased risk of PIH (odd ratio (OR) 0.7, 95% confidence intervals (CI) 0.5-1.0). The protection increased with number of cigarettes smoked per day, the OR of PIH being, respectively, 0.8 and 0.6 in women reporting <15 and > or =15 cigarettes per day at conception. The inverse relation was also observed when the analysis was conducted in strata of age, parity and nausea. Women who had quit smoking 1 year or more before conception were not at decreased risk (OR 1.0, 95% CI 0.6-1.5). No association emerged considering cigarettes smoked during the first trimester of pregnancy only. No relationship emerged between partner's smoking and risk of PIH. CONCLUSIONS This study confirms that current smokers are at decreased risk of PIH, but indicates that a reduction in risk is not present in ex-smokers.