A population-based case-control study was performed to investigate etiologic factors for wheezing bronchitis and asthma in children up to four years of age. A total of 199 children hospitalized for the first time with these diagnoses at a major hospital in Stockholm in 1986-1988 constituted the cases, 351 children from the catchment area of the hospital were used as controls. Information on known and suspected risk factors was obtained through home interviews with a parent. Parental smoking was associated with a relative risk of 1.8 (95% confidence interval 1.3-2.6) corresponding to a population attributable proportion of 27%. The strongest association was seen for maternal smoking and children below 18 months of age. Other major risk factors included atopic heredity, recurrent upper respiratory tract infections and breast-feeding less than 3 months, which appeared to interact multiplicatively with parental smoking. The environmental factors had a stronger influence in the youngest age group, and the overall attributable proportion associated with parental smoking, short breast-feeding period and exposure to pets in the household was 43%. It is clear that successful primary prevention could dramatically reduce the incidence of wheezing bronchitis in children.