The effect of parental smoking was assessed in 94 consecutively observed children, aged 7 to 17 years, who had a history of asthmatic wheezing. The 24 children whose mothers smoked, when they were compared with children whose mothers did not smoke, had 47% more symptoms, a 13% lower mean FEV1 percent, a 23% lower mean FEF25-75%, and fourfold greater responsiveness to aerosolized histamine. A dose response was evident. There was a highly significant correlation between the results of the tests and the number of cigarettes the mother smoked while she was in the house. The differences between the children of smoking and nonsmoking mothers were greater in older than in younger subjects. The smoking habits of the father were not correlated with the severity of the child's asthma.