A total of 1167 infants were followed for 1 year in a population-based prospective study to assess the effect of environmental factors on the development of allergic disorders. Some of these environmental factors are interdependent. Mothers who formula fed their infants smoked more often (p less than 0.001) and tended to belong to lower social classes (p less than 0.01). Logistic regression analysis was performed to adjust for these confounding variables. Maternal smoking adversely affected the prevalence of asthma (p = 0.003) defined as three or more separate episodes of wheezing and total allergy (p = 0.02). Infants in lower socioeconomic groups developed asthma significantly more often (p = 0.03) than infants born in higher socioeconomic groups. There was a nonsignificant trend for infants born in summer to develop asthma more than infants born in winter (p = 0.08). No effect of these factors was observed on eczema, food intolerance, or on the subgroup of infants with definite allergy (clinical disorder with positive skin prick test [SPT]). Exposure to animal dander did not influence the prevalence of clinical disorder, but positive SPT reaction to cat dander was more prevalent in infants who were exposed to cats and/or dogs (p = 0.04). Positive SPT to house dust mite occurred significantly more often in infants who were formula fed (p = 0.05). The environmental factors had a profound effect on the prevalence of asthma but not on other allergic disorders.