Diagnostic work on the detection of Chlamydia trachomatis infections in Uppsala county, Sweden, during the period from April 1985 to March 1991 was studied. The number of patients examined for C. trachomatis in this area increased annually by 6% to 15% during the study period, reaching 17,224 test occasions by the last 12-month period. A 40% decrease (from 1,412 to 852; or from 565 to 326 cases per 100,000 inhabitants) in diagnosed C. trachomatis infections was noted between the first and the last 12-month period. No significant tendencies were evident in either age distribution or sex ratio in the detected cases during the study period. Intensified sampling work was noted at youth clinics and maternal health care centers, while the dominant role of the venereal disease clinic as case detector declined steadily. The results have been interpreted in the context of available national figures for Sweden.