We assessed the relation between maternal respiratory problems and preterm labor and delivery in a cohort of 3,891 women who delivered a singleton livebirth. Study participants were asked about their medical history in the 12 months before conception; women who reported a history of asthma had a higher risk of preterm labor [relative risk (RR) estimate = 2.33, 95% confidence interval (CI) = 1.03-5.26]. Asthmatic women also had an increased risk of preterm delivery (RR = 1.77, 95% CI = 0.60-5.24). Maternal respiratory problems during pregnancy were also associated with an increased risk of both inhibited preterm labor (RR = 2.14, 95% CI = 1.25-3.66) and preterm delivery (RR = 2.03, 95% CI = 1.08-3.82). Birthweight appeared to be unrelated to maternal history of asthma. Logistic regression models that adjusted for potential confounders did not appreciably alter the results. The adjusted odds ratio (OR) estimate for the odds of preterm labor in asthmatics relative to nonasthmatics is 2.42 (95% CI = 0.90-6.54); the analogous OR estimate for those with respiratory problems during pregnancy is 2.16 (95% CI = 1.14-4.10). The adjusted OR estimate for the odds of preterm delivery in asthmatics is 1.78 (95% CI = 0.53-6.02); for those with respiratory problems during pregnancy, the corresponding estimate is 2.00 (95% CI = 0.92-4.13).