Specific immunotherapy has been successfully used in the treatment of allergic diseases for years. In this study, we examined the clinical efficacy of specific immunotherapy and its effect on cysteinyl leukotriene releasability by blood leukocytes in house dust mite allergic subjects with asthma and allergic rhinitis. In an open, parallel study, 13 subjects were treated with specific immunotherapy for 12 months and five patients served as the control group. Before specific immunotherapy treatment, and 1 and 12 months after, patients were evaluated with respect to total immunoglobulin (Ig) E levels, Dermatophagoides pteronyssinus specific IgE levels, symptom scores, usage of rescue medicines, lung function tests, nasal challenge scores, skin reactivity to D. pteronyssinus and cysteinyl leukotriene releasability by blood leukocytes. At the 12th month of specific immunotherapy, total asthma symptom scores, bronchodilator requirement, number of sneezes following nasal challenge and immediate skin reactivity to D. pteronyssinus was decreased, while FEV1 increased compared to the pretreatment values (p < 0.05 for each). These parameters remained unchanged in the control group. In vitro cysteinyl leukotriene releasability by blood leukocytes in response to D. pteronyssinus antigen and anti-IgE antibody remained unchanged both in the specific immunotherapy group and the control group. However, in the treated group, there were six patients who showed at least a 50% reduction in their symptoms after specific immunotherapy. In five of these, cysteinyl leukotriene release induced by both HACM buffer and D. pteronyssinus were decreased by more than 50% of the baseline values. The other patient demonstrated only a marked decrease (more than 50%) in background. Our results suggest that specific immunotherapy is effective in the treatment of allergic rhinitis and bronchial asthma due to house dust mite allergy and that the clinical response to specific immunotherapy may be associated with decreased cysteinyl leukotriene releasibility by blood leukocytes in some patients.