Parental histories of childhood respiratory trouble (CRT) were examined as risk factors for lower respiratory tract illnesses in healthy infants enrolled in the Children's Respiratory Study, Tucson, Arizona. A parental history of childhood respiratory trouble before age 16 was a risk factor for infantile lower respiratory tract illnesses (LRIs). Early age of onset of the parental CRT and those illnesses described as asthma or bronchiolitis showed the greatest risk: odds ratio = 2.8, P < 0.05. After controlling for known and suspected confounders, a parental history of CRT described as asthma or bronchiolitis with onset before age 3 was associated with wheezing LRIs in their children, with an odds ratio of 2.6, P < 0.05. A parental history of CRT described as bronchitis/croup was associated with nonwheezing LRIs in their children: odds ratio = 2.2, P < 0.05. These findings suggest a familial component to childhood respiratory trouble which may have a hereditary basis.