PURPOSE: The sight-threatening complications of trachoma are trichiasis and corneal opacity, and these remain the world's most common cause of preventable blindness. The aim of the study was to investigate the rate of progression of trachomatous conjunctival scarring to trichiasis and corneal opacity and to investigate risk factors for such progression. DESIGN: A 12-year, longitudinal study of the incidence and risk factors for the development of trichiasis and corneal opacity in a cohort of patients initially identified with trachomatous scarring in The Gambia. PARTICIPANTS: Six hundred thirty-nine subjects with some degree of trachomatous scarring were identified from the 1986 survey, and attempts were made to trace all these subjects. METHODS: Successfully traced subjects were interviewed, examined, and graded for trachoma. MAIN OUTCOME MEASURES: (1) Twelve-year rates of progression. (2) Risk factors for progression of disease. RESULTS: Three hundred twenty-six of six hundred thirty-nine (51%) subjects were traced and examined, 108 (17%) had died, and 205 (32%) were lost to follow-up. After 12 years, 6.4% (95% confidence interval [CI], 4.0-9.97) of scarred subjects had trichiasis develop, 5.96% (95% CI, 3.67-9.42) had corneal opacity develop, 16.51% (95% CI, 12.71-21.13) had visual impairment/blindness develop, and 2.5% (95% CI, 1.2-5.0) had corneal visual impairment/blindness develop. Mandinka ethnicity was a risk factor for trichiasis (odds ratio [OR], 4.3; 95% CI, 1.3-14.4), and trichiasis at baseline was a risk factor for corneal opacity (OR, 8.4; 95% CI, 1.8-39.2). History of lid surgery for trichiasis was associated with corneal opacity at follow-up (OR, 4.4; 95%CI, 1.4-14.0). Older age was a significant risk factor for development of trichiasis, corneal opacity, and visual loss (OR, 1.07; 95% CI, 1.01-1.12). Bilateral cataract was present in 40% of traced subjects and was associated with the incidence of visual impairment/blindness (OR, 9.4; 95%CI, 4.5-19.6) CONCLUSIONS: This is the first study to demonstrate the link between trichiasis and future corneal opacity, and it provides the rationale for performing lid rotation surgery on patients with trichiasis who do not yet have corneal opacity. The association between corneal opacity at follow-up and previous surgery among trichiasis patients suggests late presentation as a problem. The planning of surgical services will be aided by the incidence figures generated by this study.