The prevalence of antibodies to Chlamydia was determined in 9 population groups with different risk levels for sexually transmitted diseases in Belém, Pará state, Brazil. 583 serum samples were examined by the indirect immunofluorescence test. The prevalence rates varied from 33.3% in patients attending a sexually transmitted disease clinic to 97.1% in Parakana Indians. One or more pathogens were found in 55 of 86 patients presenting a clinical picture of urethritis; C. trachomatis was isolated from 11% (6/55) of these cases by inoculation to McCoy cell cultures, accounting for 30% (6/20) of the cases of non-gonococcal urethritis. C. trachomatis was isolated from one of 28 female patients (3.6%) attending gynaecology and obstetrics outpatient clinics. The positive patients had no symptom or complaint that would have suggested the diagnosis.