The prevalence of sensitization to the storage mites Acarus siro (AS), Tyrophagus putrescentiae (TP), and Lepidoglyphus destructor (LD) was studied in 250 sera of patients with different degrees of sensitization to the house-dust mite Dermatophagoides pteronyssinus (DP) by measuring IgE binding to extracts of the storage mites. Additionally, allergenic cross-reactivity between DP and the storage mite species was studied by RAST inhibition with five individual sera (and a pool of these sera) with moderate IgE levels to all three storage mites and to DP. Increased serum IgE to storage mites was found in 46% of the 200 patients sensitized to DP. Increased prevalence rates of IgE titers to storage mites were associated with higher IgE levels to DP. In 50 sera without sensitization to DP, only five sera showed increased IgE to one of the storage mites. Extracts of TP almost completely inhibited the IgE binding to AS, and vice versa. DP inhibited IgE binding to all storage mites up to 60%, whereas IgE binding to DP was only minimally inhibited by extracts of storage mites. In conclusion, cosensitization to storage mites is a frequent finding in patients sensitized to DP. Although this is largely the result of cross-reactivity between different mite species, it may nevertheless be of clinical significance in patients exposed to storage mites.