To identify modifiable risk factors for wheezing illness in childhood, the associations between current asthma or wheezing and factors such as household smoking, damp and dietary salt preference were measured in a questionnaire-based prevalence study of schoolchildren 7 to 9 yr of age in Cape Town. In a random sample of 15 schools, questionnaires were completed by parents of 1,955 children, from which 368 cases and 294 controls were selected on the basis of reported asthma diagnosis or symptoms. Urinary cotinine concentrations were measured, and the parents were interviewed. An exposure-response relationship between the urinary cotinine creatinine ratio and asthma/wheeze was observed. In multivariate analysis, predictors of asthma/wheeze were hay fever (odds ratio [OR] - 5.30; 95% confidence interval [CI] = 3.16 to 8.89), eczema (OR = 2.19; 95% CI = 1.33-3.62), parental asthma (OR = 1.77; 95% CI = 1.11 to 2.84), absence of paternal contribution to income (OR = 1.72; 95% CI = 1.17 to 2.54), maternal smoking in pregnancy (OR = 1.87; 95% CI = 1.25 to 2.81), and each additional household smoker (OR = 1.15; 95% CI = 1.01 to 1.30). Findings were similar, with higher odds ratios for most variables, except number of household smokers, when the group was restricted to children with parent-reported asthma. The findings confirm that household smoking is an important modifiable risk factor in asthma/wheeze among young schoolchildren, and they suggest that maternal smoking in pregnancy and current household exposure are independent contributors to this effect.