To evaluate the prevalence of genital infection with Chlamydia trachomatis in different groups of patients in the region of Plovdiv, Bulgaria, over a period of four years (1992-1995) and to establish epidemiological and clinical correlates of infection. Five groups were enrolled in the study: group A-990 symptomatic male sexually transmitted disease (STD) patients, group B-1023 symptomatic female STD patients, group C-253 gynaecological patients, and asymptomatic groups D-276 men and group E-231 women. Urethral samples from men and cervical and urethral samples from women were examined for C. trachomatis antigen by direct immunofluorescence assay (DFA) or enzyme immunoassay (EIA) confirmed by DFA at the STD outpatient clinic of the Medical Institute in Plovdiv. The prevalence rates for genital infection with Chlamydia trachomatis were 25.1% in group A, 21.8% in group B, 15.4% in group C, 4.3% in group D and 6.1% in group E. Infection with C. trachomatis was more common in patients who were < or = 25 years of age, unmarried, and with risk sexual behaviour, as well as in those who had signs and symptoms suggestive of infection. The study shows a relatively high prevalence of infection with C. trachomatis in STD and gynaecological patients. Symptoms and signs correlated with C. trachomatis infection, but were not specific or reliable indicators of infection. C. trachomatis detection should be included in standard STD and gynaecological examination.