The object of this study was to determine the relative importance of low birth weight, preterm birth, low maternal age, household size, exposure to maternal smoking, personal smoking at 16 yrs of age, early termination of breastfeeding and socioeconomic status in the aetiology of wheezing illness in the first 5 yrs of life, and on the persistence of this illness at 16 yrs of age. In 15,712 children born in Britain during one week of April 1970, the occurrence of wheezing by 5 yrs of age, and of wheezing in the past year at 16 yrs of age within this group were analysed in multivariate logistic regression against each potential risk factor. The independent determinants of wheezing by 5 yrs of age were male sex, maternal smoking during pregnancy (odds ratio (OR) for 15+ cigarettes.day-1 = 1.39; 95% confidence interval (95% CI) 1.22-1.58) and low birthweight (OR for birthweight < 2.5 kg = 1.26; 95% Cl 1.07-1.50). Of children who had wheezed by 5 yrs of age, 15% reported wheezing in the past 12 months at 16 yrs of age. The persistence of symptoms at 16 yrs of age was independently related to low maternal age (OR for 20 vs 40 yrs of age = 1.96; 95% CI 1.08-3.45) and to high social status (OR for most vs least advantaged = 1.95; 95% CI 1.13-3.38). We conclude that low birth weight and maternal smoking in pregnancy are independent risk factors for early childhood wheezing, but in 85% of children with early wheezing it resolves by 16 yrs of age.(ABSTRACT TRUNCATED AT 250 WORDS)