SUMMARY At present there are no reliable statistics on the relative prevalences of sexually transmitted diseases (STDs) in Spain. In a report of the first three years' experience in an STD diagnostic centre between 1977 and 1979 a total of 879 patients (534 men and 345 women) were seen. They mainly consisted of university students and the mean age was 22 years in 1977 and 23 years in the following two years. All the patients were examined for syphilis and all women for gonorrhoea and trichomoniasis. Investigations for Chlamydia trachomatis, Mycoplasma hominis, Ureaplasma urealyticum, Candida albicans, and Herpesvirus hominis infections were carried out according to the presenting symptoms. Non-specific genital infections occurred most commonly (25-70o); chlamydia were isolated from 30% of the patients with non-gonococcal urethritis (NGU). The second commonest infection was candidosis (13-5%). Gonorrhoea, which was found in 10 607o of the patients, was diagnosed more frequently in men (13' 5'%) than in women (6%).No strains of P-lactamase-producing Neisseria gonorrhoeae were detected and all were sensitive to penicillin. Syphilis was diagnosed in 4-4'0 of patients (2% women and 5% men). Condylomata acuminata were diagnosed in 2-8% of patients and more frequently in men (4Wo). Herpes genitalis and venereophobia were uncommon (Il 9% and 1-20o respectively) and were diagnosed only in men.