The clinical features of perennial asthmatic children with a skin or bronchial reaction to the house dust mite (Dermatophagoides pteronyssinus) were compared with those of asthmatic children without mite sensitivity. Mite sensitive asthma was characterised by an early age of onset of symptoms, these being predominantly nocturnal. A history of wheezing precipitated by dust exposure, during vacuuming, bedmaking, or dusting was present in 52% of cases. Asthmatic children with mite sensitivity were more likely to have been born at the time of the year when mite counts were highest. This was consistent with the idea that allergy may be associated with a period of susceptibility to sensitisation in early infancy.