Postneonatal mortality due to respiratory illnesses is known to be inversely related to maternal age, but the possible role of young motherhood as a risk factor for respiratory morbidity in infants has not been thoroughly explored. The authors studied the incidence of lower respiratory tract illnesses during the first year of life, as ascertained by health plan pediatricians, in over 1,200 infants enrolled at birth between 1980 and 1984 in Tucson, Arizona. The incidence of wheezing lower respiratory tract illnesses increased significantly (p = 0.005) with decreasing maternal age, whereas the incidence of nonwheezing lower respiratory tract illness was independent of maternal age. A logistic regression was used to control for the effects of several known confounding factors. When compared with infants of mothers aged more than 30 years, adjusted odds ratios were 2.4 (95% confidence interval 1.8-3.1) for infants whose mothers were less than age 21 years (p < 0.0001), 1.8 (95% confidence interval 1.4-2.3) for infants whose mothers were aged 21-25 (p < 0.0001); and 1.4 (95% confidence interval 1.1-1.6) for infants whose mothers were aged 26-30 (p < 0.001). These results suggest that young motherhood is an important risk factor for wheezing lower respiratory tract illnesses during the first year of life. Both biological and social factors related to maternal age may explain these findings.