BACKGROUND: Trachoma, caused by repeated ocular infection with Chlamydia trach omatis, is an important cause of blindness. Current recommended dosing intervals for mass azithromycin treatment for trachoma are based on a mathematical model. METHODS: We collected conjunctival swabs for quantitative polymerase chain re action assay of C. trachomatis before and 2, 6, 12, 18, and 24 months after mass treatment with azithromycin in a Tanzanian community in which trachoma was ende mic. For ethical reasons, at 6, 12, and 18 months, we gave tetracycline eye oint ment to residents who had clinically active trachoma. RESULTS: At baseline, 956 of 978 residents (97.8 percent) received either one oral dose of azithromycin or (if azithromycin was contraindicated) a course of tetracycline eye ointment. Th e prevalence of infection fell from 9.5 percent before mass treatment to 2.1 per cent at 2 months and 0.1 percent at 24 months. The quantitative burden of ocular C. trachomatis infection in the community was 13.9 percent of the pretreatment level at 2 months and 0.8 percent at 24 months. At each time point after baselin e, over 90 percent of the total community burden of C. trachomatis infection was found among subjects who had been positive the previous time they were tested. CONCLUSIONS: The prevalence and intensity of infection fell dramatically and rem ained low for two years after treatment. One round of very high cover age mas s treatment with azithromycin, perhaps aided by subsequent periodic use of tetra cycline eye ointment for persons with active disease, can interrupt the transmis sion of ocular C. trachomatis infection. BACKGROUND: Trachoma, caused by repeated ocular infection with Chlamydia trach omatis, is an important cause of blindness. Current recommended dosing intervals for mass azithromycin treatment for trachoma are based on a mathematical model. METHODS: We collected conjunctival swabs for quantitative polymerase chain re action assay of C. trachomatis before and 2, 6, 12, 18, and 24 months after mass treatment with azithromycin in a Tanzanian community in which trachoma was ende mic. For ethical reasons, at 6, 12, and 18 months, we gave tetracycline eye oint ment to residents who had clinically active trachoma. RESULTS: At baseline, 956 of 978 residents (97.8 percent) received either one oral dose of azithromycin or (if azithromycin was contraindicated) a course of tetracycline eye ointment. Th e prevalence of infection fell from 9.5 percent before mass treatment to 2.1 per cent at 2 months and 0.1 percent at 24 months. The quantitative burden of ocular C. trachomatis infection in the commu nity was 13.9 percent of the pretreatment level at 2 months and 0.8 percent at 24 months. At each time point after baselin e, over 90 percent of the total community burden of C. trachomatis infection was found among subjects who had been positive the previous time they were tested. CONCLUSIONS: The prevalence and intensity of infection fell dramatically and rem ained low for two years after treatment. One round of very high cover age mas s treatment with azithromycin, perhaps aided by subsequent periodic use of tetra cycline eye ointment for persons with active disease, can interrupt the transmis sion of ocular C. trachomatis infection.