Analysis of the month of birth of 1715 children with asthma shows clear evidence of a higher proportion of the group having been born between May and October (54.8%) than would be expected by comparison with the population of England and Wales (50.2%). Inspection of the distribution of month of patients with hay fever and for patients with positive skin reactions to mixed grass pollen does not account for this seasonal variation but children with positive skin reactions to Dermatophagoides pteronyssinus show a high proportion (56.2%) born in the period May to October. Although the reasons for the variation of pattern of month of birth in asthmatic children are uncertain, further study of sensitization in early life might clarify them.