Aim: To analyse the importance of mothers' smoking during pregnancy and/or environmental tobacco smoke (ETS) exposure in early childhood for children's health and well-being at the age of 3 years. Methods: Four groups from a population based cohort (n = 8850) were compared: children with nonsmoking mother during pregnancy and nonsmoking parents at the age of 3 years (n = 7091), children with only foetal exposure (n = 149), children exposed only postnatally (n = 895) and children exposed both pre- and postnatally (n = 595). Odds ratios and 95% confidence intervals were calculated. Results: Children exposed both pre- and postnatally had more wheezing (1.14, 1.07-1.21) and rhinitis (1.16, 1.06-1.26), used more cough-mixture (1.07, 1.01-1.14) and broncodilatating drugs (1.08, 1.02-1.15) and suffered more from excessive crying (1.31, 1.13-1.51) and irritability (1.27, 1.09-1.48) compared to children with nonsmoking parents. Children exposed only postnatally had more rhinitis (1.24, 1.12-1.37), used more cough-mixture (1.14, 1.05-1.29) and suffered more from poor sleep (1.26, 1.07-1.47) than children of nonsmoking parents. Children with prenatal exposure only used more broncodilatating drugs (1.45, 1.03-2.04) and suffered more from poor sleep (2.06, 1.09-3.87). Conclusion: Health differences, small but significant, indicate that prenatal and/or postnatal ETS exposure alone, or in combination, seems to interfere with child health, supporting the importance of zero tolerance. However, as most smoking parents in Sweden try to protect their children from ETS exposure, the results also might indicate that protective measures are worthwhile. © 2007 The Author(s).