The objective of the study was to assess the prevalence of passive smoking in infancy. This was done by self-report questionnaires completed by parents who attended the well-baby clinic in the period February-May 1996. A total of 2720 questionnaires were spread among parents with babies between 1 and 14 months: smoking and non-smoking parents. The questionnaires contained questions on smoking habits, smoking at home, smoking in presence of the baby. A total of 1702 parents filled in and returned the questionnaire (63%); 24% of the mothers and 33% of their partners smoked. In 44% of the families, one or more persons smoked; 22% of the mothers and 26% of the partners smoked at home. In 39% of the families, one or both parents smoked at home; 42% of the babies were exposed to tobacco smoke in the living-room, 8% were exposed in the car, and 4% during feeding. In cases where only the mother smoked, 13% of the infants were exposed to tobacco smoke during feeding. In the families where only the partner smoked, the babies were predominantly exposed to smoke in the car (18%). If both parents smoked, the child was most frequently exposed to tobacco smoke in the living-room (73%). It can be concluded that health workers, nurses, pediatricians and family physicians should be advised to inform parents systematically of the harmful effects of passive smoking in infancy. If parents are unable or unwilling to stop smoking, it is important to advise them to refrain from smoking in the presence of the baby.