Adult Asian immigrants to the United Kingdom attending an asthma clinic have been compared with a control group of non-immigrant Caucasian asthmatic patients of similar age distribution. The Asian immigrants had a later age of onset of asthma than their non-immigrant controls. Comparison with studies of asthmatic patients in India suggests that this may be an intrinsic ethnic difference but an effect of migration in early adult life is not excluded. Despite a later age of onset, the frequency of positive skin prick tests to common allergens was similar in the immigrant and control groups and 71% of the Asians had positive reactions to the house dust mite. Most other clinical features, the variability of airways obstruction and the requirements for treatment showed no significant differences between the two groups.