OBJECTIVE To quantify the combined effects of systolic blood pressure (SBP) and cigarette smoking on incident coronary heart disease (CHD) in women. METHODS Overall, 86,338 women aged >or=40 years were enrolled in 1991. The follow-up evaluation was conducted in 1999-2000, with a response rate of 92.9%. RESULTS A total of 829 CHD events (fatal and nonfatal) were observed among the participants who were free of cardiovascular diseases (CVD) at baseline. Higher SBP was significantly associated with more risk of CHD in both nonsmokers and current smokers (all p < 0.0001 for linear trends). Comparing with never smoking, both low and high levels of cigarettes smoked per day (1-7, and >or=8 cigarettes per day) and pack-years (<10, and >or=10 pack-years) were associated with increased risk of CHD in those with normal and high SBP. The multivariate adjusted relative risks (RRs) of CHD were 2.54 (95% confidence interval [CI] 2.00-3.23), 1.28 (1.01-1.63), and 1.57 (1.33-1.86) for current smokers with high SBP, current smokers with normal SBP, and nonsmokers with high SBP, respectively, compared with nonsmokers with normal SBP. The present study identified a statistically significant additive interaction between these two factors on CHD. CONCLUSIONS Our study indicated that the combined effects of cigarette smoking and high SBP could be expected to have extra adverse effects on CHD in women, which highlights the importance of multifactorial interventions to decrease the risk of CHD, for example, quitting smoking and treatment of high blood pressure in Chinese women.