The relationship between genetically determined polymorphic metabolism and susceptibility to allergic diseases has aroused much interest. The aim of our study was to evaluate whether patients with allergic diseases, like atopic asthma and allergic rhinitis differ from healthy persons in their CYP2D6 genotype. Study completed 400 persons, 100 patients with allergic diseases--62 with atopic asthma and 38 with allergic rhinitis and 300 healthy volunteers as a control group. The results of our study revealed a statistically significant predominance of extensive metabolizers among patients with allergic diseases in comparison to healthy volunteers. Relative risk (odds ratio) of development of allergic diseases was 1.85 times higher (p < 0.05), atopic asthma was 1.97 times higher (p < 0.05), allergic rhinitis 1.68 times higher (p > 0.05; NS) for persons with extensive CYP2D6 genotype. Our results represent some evidence for a possible relationship between extensive CYP2D6 genotype and the higher susceptibility to development of allergic diseases.