A comparison was made between the birth month of a control population and a sample of 240 adult patients with bronchial asthma or rhinitis and positive skin test towards house dust mite (Dermatophagoides pteronyssinus). The patients were born more frequently (P less than 0.005) in the summer and autumn months than in the other seasons. A similar comparison of 336 allergic patients with a positive skin test towards grass pollen (Phleum pratense) but no consistent seasonal preference in the birth months was revealed. This was possibly explained by the age of the patient group studied. The increased incidence of house-dust mite allergy in patients born in the months of May to September inclusive when house dust mites are supposed to be most abundant corresponds to a relative risk of 1.44. It is important to diminish the exposure to house dust mites in early childhood because exposure to allergens may influence the development of allergic disease in later life.