The former and current smoking habits of 159 patients treated for infertility by means of IVF/ET technique were observed. The data were verified by a cotinine examination in urine. The basal levels of gonadotropines before treatment, the process and outcome of ovarian stimulation, oocyte fertilisation and incidence of pregnancy were observed. There were no differences between smokers and non-smokers in terms of their age and professional risks; smoking correlated with a lower level of education. The overall response of active smokers to hormonal stimulation was worse than that of non-smokers: a lower number of mature follicles (12.3 vs. 16.2) and a lower number of oocytes were gained (7.3 vs. 10.9). There was a correlation between the age of the smokers and the higher consumption of FSH needed for stimulation and lower levels of 17 beta-estradiole. The number of fertilised oocytes in smokers was significantly lower (p < 0.01; 68.2% vs. 47.8%). A negative correlation between the duration of exposure to cigarette smoke and fertilisation rate was shown (p < 0.05). There were fewer embryos in smokers as compared with non-smokers (3.3 vs. 4.7). A total of 35 women became pregnant (i.e. 22.1%), of which 28.8% were non-smokers, 12.5% were occasional smokers, and none were regular smokers. The OR value in non-smokers was 1.48, and in occasional smokers 0.57. In this study, the negative influence of smoking on the outcome of treatment by IVF/ET technique was proved. The phasing out of smoking should be an integral part of human infertility treatment.