A prospective cohort study of 222 consecutive couples undergoing 297 cycles of in-vitro fertilization and embryo transfer (IVF-ET) was conducted to evaluate the impact of cigarette smoking in males and females. Compared with non-smokers, females smoking at the time of treatment had more previous pregnancies (1.16 versus 0.63, P less than 0.001), consumed more coffee per day (3.29 versus 1.85 cups, P = 0.001) and were less likely to hold a professional or skilled job (41% versus 66%). There was no difference in the response to ovarian stimulation in terms of the duration and dose of human menopausal gonadotrophin, peak oestradiol level or number of oocytes retrieved. The fertilization rate was actually higher in heavy smokers than in non-smokers (79.3% versus 61.3%, P = 0.007). The rate of embryo cleavage was retarded in a dose-dependent fashion. In smokers of 1-14 cigarettes/day, the likelihood of transferring an embryo at greater than or equal to 4-cell stage was 0.87 [95% confidence limits (CL) 0.56-1.4] and in smokers of greater than or equal to 15 cigarettes/day, the likelihood was 0.52 (95% CL 0.31-0.88). However, evaluation of interrelated factors using logistic regression suggested that a low socioeconomic status had a greater detrimental effect on embryo cleavage rate than female smoking. No significant difference was noted in the clinical outcome following embryo transfer. A study of larger sample size is required to evaluate whether the effects of cigarette smoking are independent of socioeconomic status and other related factors and whether they result in reduced ongoing clinical pregnancy and live birth rates.