AIM: The aim of the study was to evaluate the reliability of self-reported smoking status and environmental tobacco smoke exposure (ETS) during pregnancy, assessing serum cotinine level. METHODS: The prospective cohort study was conducted in 2 antenatal care units in Lodz, Poland. Study population consisted of 183 pregnant women between 20-24 weeks of pregnancy. All of the women who agreed to participate in the study were interviewed to investigate certain socio-demographic, lifestyles, behavioural characteristics and obstetric background. Self-reported smoking status and ETS exposure were verified with the help of serum cotinine level. Cotinine level was analyzed by means of gas chromatography with mass spectroscopy (GC-MS). We choose more than 15 ng/ml as serum cotinine level for smokers, 2-15 ng/ml for ETS exposure and less than 2 ng/ml for non-smokers not exposed to ETS. RESULTS: Among non-smoking and not ETS-exposed women, 17% had cotinine level indicating active smoking and 74% ETS exposure. About 4% of the women who indicated ETS exposure during pregnancy had serum cotinine level higher than 15 ng/ml indicating active smoking. CONCLUSIONS: The information about active and passive smoking during pregnancy obtained from mothers and based on the questionnaire does not indicate objective maternal exposure to tobacco smoke.