Pairwise analysis of siblings from 21 families showed that house dust mite (HDM) sensitive children were exposed to higher concentrations of Der p I allergen in their mattress (P= 0.005) and bedding (P= 0.04), but not bedroom floor (P= 0.33), than their atopic sibling who was not sensitive to HDM antigens. There was no difference in the exposure to HDM numbers/100 mg of dust in the mattress (P= 0.61) or bedroom floors (P= 0.09). In contrast, pairwise analysis of siblings from 15 families showed that HDM sensitive children were not exposed to significantly different concentrations of Der p I in the mattress (P= 0.96), bedding (P= 0.11) or bedroom floor (P= 0.70) nor HDM numbers/100 mg of dust in the mattress (P= 0.12) and bedroom floor (P= 0.98) than their non-atopic siblings. These findings were identical when absolute allergen load was compared in these pairs. Genetic linkage studies in these families suggest the tendency to atopic IgE responses is conferred by a putative atopy locus on chromosome 11q. These results together suggest that differences in allergen levels in beds, among siblings with a comparable genetic tendency to atopy, play a significant role in determining the development of HDM allergy.