BACKGROUND: Data from studies done in communities where trachoma is mesoendemic suggest that ocular infection with Chlamydia trachomatis can be eliminated after one mass treatment with antibiotics. However, there are no comparable long-term data from trachoma hyperendemic communities. Our aim, therefore, was two-fold: first, to ascertain the disease pattern of trachoma and ocular infection with C trachomatis in a trachoma hyperendemic community after mass treatment; and, second, to ascertain the risk factors for incident infection. METHODS: We did a longitudinal study of a trachoma hyperendemic community (n=1017) in Tanzania. We did surveys, including ocular swabs, at baseline, 2, 6, 12, and 18 months to identify the presence, and quantity, of C trachomatis after single mass treatment of all individuals aged 6 months or older with azithromycin 20 mg per kg; pregnant women without clinical disease received topical tetracycline. FINDINGS: Mass treatment (coverage 86%) significantly reduced the prevalence of infection from 57% (495 of 871) to 12% (85 of 705) at 2 months. Infection remained fairly constant to 12 months, with evidence of increasing numbers and load of infection by 18 months post-treatment. Incident infection at 6 months was 3.5-times more likely if another member of the household had more than 19 organisms per swab at 2 months. Travel outside the village, and visitors to the household, did not increase the risk of infection within households up to 12 months. INTERPRETATION: In this trachoma hyperendemic community, infection levels after high antibiotic coverage persisted at a low level to 18 months, with evidence for re-emergence after 1 year. Fairly light loads of infection were associated with household transmission. Yearly mass treatment over a few years could be sufficient to eliminate infection.