The interaction between sex and smoking habits on pulmonary function was examined among 1,149 adults 25 to 59 yr of age in a rural community in Saskatchewan. Pulmonary function tests included FVC, FEV1, maximal midexpiratory flow rate (MMFR), the slope of phase III of the single-breath nitrogen test (delta N2/L), and closing volume as a percent of vital capacity (CV/VC). The data show that after fixing the effects of age, height, and weight by analysis of covariance, the adjusted means of delta N2/L in nonsmokers, ex-smokers, and current smokers were 0.92, 1.10, and 1.60% in women and 0.97, 1.05, and 1.23% in men, respectively. The difference in the adjusted means for delta N2/L between smokers and nonsmokers was larger in women than in men, 0.67% versus 0.26%, respectively. Multiple multivariate analyses show that the regression slopes for the residuals of FEV1, MMFR, and delta N2/L versus pack-years were significantly different between men and women. The regressions of FEV1 and MMFR decreased and the regression of delta N2/L increased with increasing pack-years more rapidly in women than in men. The combined effect of sex and pack-years on pulmonary function was not significant for ex-smokers. These data suggest that cigarette smoking may be more detrimental in its effects on lung function in women than in men.