This paper summarizes primary epidemiologic studies of trachomatous blindness to develop age-/sex-/region-specific estimates of the global prevalence of trachomatous blindness and low vision. These studies are first examined for their validity and then employed to derive a 'minimum' prevalence of trachomatous visual impairment. This method yield a global total for 1990 of approximately 640,000 cases of trachomatous blindness, corresponding to a prevalence of 0.12/1,000 (lower and upper bounds, 0.10 to 0.14/1,000). When those with low vision due to trachoma are included, 1.5 million cases of visual impairment are estimated, corresponding to a prevalence of 0.28/1,000 (lower and upper bounds, 0.15 to 0.75/1,000). A second approach, labeled the 'projected' prevalence of trachomatous visual impairment, selects country-wide studies to derive representative regional prevalence values. A global total of 2,899,000 blind ("projected' prevalence of 0.55/1,000 with lower and upper bounds, 0.37 to 0.83/1,000) is estimated for 1990. With trachomatous low vision included, greater than 6.7 million individuals in 1990 have visual impairment from trachoma ("projected' prevalence of 1.28/1,000 with lower and upper bounds, 0.53 to 4.29/1,000). Analysis of the distribution of the global prevalence by age, gender, visual acuity and region provide direction for trachoma research and programme priorities. Attention is drawn to the limited studies and resulting wide variation in the estimates of trachomatous visual impairment prevalence as indicated by the lower and upper bound estimates. It is recommended that this epidemiologic uncertainty be reflected in global and regional estimates of trachomatous visual impairment prevalence in order to draw attention to how little is known and emphasize the need for further surveys. A second paper incorporates these findings in an assessment of the global burden of trachomatous visual impairment.