Lower respiratory tract illness (LRI) is associated with exposure to various environmental factors. The relation between home environment and LRI in infants was studied with the use of data from the Children's Respiratory Study in Tucson, Arizona. Healthy infants from a health maintenance organization were recruited at birth (1980-1984). Analysis was restricted to one infant per family, and to those followed through the first year (n=936). Environmental data were collected at enrollment, and clinicians diagnosed LRI according to predetermined criteria. During the first year of life, 196 infants (21%) had wheezing LRI, and 60 (6%) had nonwheezing LRI. The risk of wheezing LRI was higher in infants with evaporative home cooling (24%) than in those without evaporative home cooling (15%) (odds ratio = 1.8, 95% confidence interval 1.1-3.0); this association was stronger among infants who lived with other children in the home. The risk of nonwheezing LRI was associated with parents' rating of neighborhood dustiness, ranging from 5% in the least dusty environments to 12% in the dustiest (p for trend = 0.002). Neither association could be explained by confounding factors. LRI was not related to the type of home heating, cooking fuel, or the numbers of indoor dogs or cats.