The ETS exposure in 20 homes with and in 10 homes without smokers was assessed by questionnaires, air monitoring, personal sampling and biomonitoring. The household members kept diaries on their tobacco smoke exposure over a period of 1 week. The subjects wore personal diffusion samplers for nicotine over the same time period. In the living rooms of smoking homes in the evening when all household members were present, significantly higher median concentrations of respirable suspended particles (116 vs 67 µg/m³, UV-PM: 52 vs 3 µg/m³), nicotine (3.3 vs < 0.01 µg/m³), 3-ethenylpyridine (1.0 vs < 0.01 µg/m³) and carbon monoxide (0.8 vs 0.2 ppm) were found than in those of nonsmoking homes. The concentrations of carbon dioxide (CO₂), an indicator of the ventilation conditions, were comparable in both types of households. Non-smokers from smoking homes (N=43) reported a longer daily ETS exposure duration (medians: 2.8 vs 0.4 h) and had higher nicotine levels on their personal samplers (medians: 0.3 vs 0.07 µg/m³) compared to nonsmokers from nonsmoking homes (N=39). The biological monitoring by measuring cotinine in saliva and urine confirm these results. The individual ETS exposure markers correlated significantly with the extent of exposure determined by air monitoring at home. Our data suggest that for nonsmokers from smoking homes, about 80% of the total ETS exposure originates from smoking in the home.