This report describes a retrospective study of 1421 patients allergic to pollens and 728 patients allergic to animal epithelia. In males, but not in females, the month-of-birth distribution differed significantly from that in the total population of Finland (P < 0·01 in pollen allergy and P < 0·05 in epithelium allergy). The phenomenon was most clearly apparent in boys aged 10–14 born in 1960–4 (P < 0·01 in pollen allergy and P < 0·001 in epithelium allergy). When the relative risk of immediate hypersensitivity was plotted as a function of the month of birth, a two-phasic curve with maxima in March–May and September–November was obtained for both pollen allergy and animal epithelium allergy. The results indicate that boys, but perhaps not girls, have a sensitive period early in life during which one or more environmental factor and seasonal variations in this factor influence the development of subsequent immediate hypersensitivity.