The association between history of asthma in the mother and low birth weight (< 2,500 g) was studied in 2,929 primary schoolchildren, randomly selected from three areas of the Lazio Region, Italy, and enrolled in a cross-sectional survey to assess their health status in relation to environmental factors. A history of asthma in the mother was associated with a higher prevalence of low birth weight, with a crude OR of 2.95 (95% CI 1.10-6.72). After stratification for other variables, an association was still present only for males (OR 4.13; 95% CI 1.01-12.53), when mothers had smoked in pregnancy (OR 8.02; 95% CI 1.63-32.28) and were resident in an industrial town (OR 10.21; 95% CI 2.69-32.27). An OR of 6.43 was also found when mothers belonged to low social class, but the 95% CI included the unity. These results suggest that a history of asthma in the mother is a risk factor for low birth weight, but only when other adverse factors are concurrently present.