Sexually transmitted diseases (STD) were recently included in the Spanish reporting system, or Compulsory Reporting of Diseases (EDO). In the present study, the validity of this system is evaluated with the estimation of the positive predictive values of the EDO. A preestablished questionnaire was administered by phone to the reporting physicians of the 478 cases of STD that had been reported to the EDO system from August 1987 to May 1988 in the Alicante and Valencia provinces. In this survey, in addition to data related to the reporting physician (specialty and place of work) and to the patient (sex, clinical symptoms and signs), information permitting to classify each diagnosis as certain, likely and unlikely was collected (incubation time, laboratory tests). Considered as a whole, the results suggested an acceptable accuracy (positive predictive value 79.09%, confidence limits 76.7%-81.5%) for both diseases. This permits to consider STD statistics as fairly accurate. Among the factors influencing the diagnostic precision are the disease itself, the patient's sex, the qualification of the reporting physician and his place of work. The predictive value of the cases reported from centers of diagnosis of STD was higher. We think that, although these results are acceptable for the epidemiological survey, the quality of care could be enhanced by a better organization of services and an improved undergraduate and postgraduate education.