As part of a population-based study of diabetes mellitus, the 4-year incidence of macular edema and its relationship to various risk factors was examined in a group of younger onset insulin-taking persons (n = 610) and older onset persons (n = 652). The presence of macular edema at the baseline and follow-up examinations was determined from gradings of stereoscopic fundus photographs. The overall incidence of macular edema in the younger onset group was 8.2% (50/610); in the group of older onset persons using insulin, 8.4% (23/273) and in the group of those not using insulin, 2.9% (11/379). The incidence of macular edema was associated with higher level of glycosylated hemoglobin, longer duration of diabetes, and more severe retinopathy at the baseline examination in both younger and older onset groups. These data provide accurate population-based estimates of incidence of macular edema, and suggest that the level of glycemia is a significant risk factor for the development of macular edema.