Specific immunotherapy (SIT) using a standardized mite extract is effective and safe when administered under optimal conditions. However, the duration of its effectiveness after cessation of treatment remains unknown. Forty asthmatic subjects who had received SIT with a standardized Dermatophagoides pteronyssinus (Der p) extract under the same protocol were studied. All had received SIT for a period of 12-96 months and were not receiving pharmacologic treatment. The FEV1 was within normal range in all patients. After cessation of treatment, patients were followed for up to 3 years at 6-month intervals. The patient was considered to have relapsed when symptoms of asthma and/or rhinitis occurred and/or when pulmonary function tests were impaired. Skin tests with increasing concentrations of Der p were carried out before and at the end of SIT. Forty-five percent of the patients did not relapse. The duration of efficacy of SIT was related to the duration of SIT itself (P < 0.04). Most patients who did not relapse had a decrease in skin test reactivity at the end of SIT, whereas most patients who relapsed did not show any change (P < 0.003). The duration of efficacy of SIT after its cessation depends upon the duration of SIT and may be predicted by the effect of SIT on skin tests.