BACKGROUND Little is known about the effect of asthma on employment. The relation between employment history at the age of 23 years and a history of asthma or wheeze was investigated in a controlled prospective study using data collected in the National Child Development Study, a longitudinal survey of all children born in the United Kingdom in one week in March 1958. METHODS Information about subjects' medical condition was collected at four ages (7, 11, 16, and 23 years) for the original cohort of 17,319 births. At 23 years information about employment and education was obtained for 12,534 subjects (72%), of whom 460 (4%) had current asthma or wheeze, 2758 (22%) had past asthma or wheeze, and 5161 (41%) had never had asthma or wheeze. The remaining subjects could not be classified accurately. RESULTS The risk of unemployment was higher in subjects with a current history of asthma or wheeze (odds ratio 1.32, 95% confidence interval (CI) 1.09-1.61) or a past history of asthma or wheeze (odds ratio 1.54, 95% CI 1.27-1.85) than in those with no such history, after subjects' sex, region of birth, maximum educational qualification, and father's social class had been controlled for. Current and past asthma or wheezing illness predicted a worse employment history in terms of most of the outcomes examined, including mean percentage of months employed since leaving school, mean number of months in current full time job, mean percentage of months unemployed since leaving school, likelihood of being out of the labour force owing to long term illness, and the proportion attaining social groups 1-3. The differences from those who had never had asthma or a wheezing illness were, however, small and generally non-significant. CONCLUSION Asthma has only a small adverse effect on employment in young adults.