The objective of the study was to assess the role of smoking parents' education, socioeconomic status and knowledge of the child's health as determinants of the child's exposure to environmental tobacco smoke (ETS) at home. A population-based cross-sectional study was carried out by mailing a parent-administered questionnaire (response rate 80.3%) to a random sample of children aged 1 to 6 years, in Espoo. The present analyses focused on the 1003 children at whom one or both parents or guardians were currently smokers. A total of 253 children (25.2%) of smoking parents were reported to be exposed to ETS at home at the time when the questionnaire was filled in, while 750 (74.8%) children were not. Adjusted odds ratios (OR) for being exposed to ETS were assessed in the logistic regression. The risk of exposure to ETS was significantly higher when the parents had no professional education, compared to university or college education (OR 2.42, 95% Confidence Interval (CI) 1.43-4.11), but socioeconomic status was not associated with exposure. The risk was higher with a single parent or guardian than in families with two parents (2.17, 1.36-3.44). The risk decreased significantly for atopic children (0.61, 0.38-0.98), indicating that knowledge of the child's illness affects the smoking behavior of the parents.