The distribution by month of birth among 7796 patients, who consulted Danish general practitioners during a one year period 1977-78 because of asthma and/or allergic rhinitis is presented. Among allergic rhinitis patients births in March and April were significantly overrepresented, and July underrepresented. As a whole February-May were overrepresented and July-January underrepresented. The distribution of month of birth of asthma patients did not differ from the distribution of the total Danish population. Patients suffering from both asthma and allergic rhinitis presented a pattern similar to that of allergic rhinitis patients. Families with a high prevalence of allergy may take advantage of this knowledge when planning child births, as the findings may be explained partly by exposure to pollen during infancy.