The month of birth distribution for 1301 French patients born between 1953 and 1975 with at least one positive skin test was compared to that of the whole population. A chi-square test was performed, and the expected birth month distribution of the groups calculated from the INSEE* data for 1953-1975. All patients underwent skin testing with house dust, Dermatophagoides pternyssinus (Dpt), cat and dog allergens, grass, tree and weed pollens, and moulds. The only study criterion was a positive skin test unrelated to any specific disorder. A significant difference in month of birth distribution was observed 1) for patients with positive skin test to grass pollen, with a high rate of births from January to May, and 2) for patients with mould sensitization, with a low rate of births in April, May and December. Tree and weed pollens, house dust and Dpt showed no significant relation with month of birth. For cat and dog allergens, the observed and expected distributions of birth month were similar. For the whole sensitized population the birth rate tended to be low in December except for the cat and dog sensitized. Our study confirms the well-known seasonal peak of births in the first 5 months of the year for grass pollen sensitized patients. No consistent monthly or seasonal tendency could be statistically demonstrated for other allergens except moulds.