PURPOSE: To develop a severity scale for diabetic macular edema (DME) and to assess relationships between severity and duration of DME and visual acuity (VA). METHODS: From the Early Treatment Diabetic Retinopathy Study (ETDRS), mean baseline VA scores were tabulated for 7422 eyes cross-classified by (1) location of retinal thickening (RT) and its area within 1 disc diameter of the macular center, and (2) degree of RT at the center. Adjacent (row, column, and off-diagonal) cells with the greatest similarity in baseline VA (mean and SD) based on a Gaussian (normal) likelihood were merged. An initial eight-step scale was chosen using the Schwarz criterion (Bayesian information criterion; BIC) and was revised based on clinical judgment to nine steps. Relationships between baseline VA and other photographic and fluorescein angiographic characteristics were examined singly and in combination with the scale. RESULTS: Modeling baseline VA as a function of the nine-step scale yielded an R(2) of 38.0%, compared with 38.4% using the full cross-classification of these variables. Addition of each of the other baseline characteristics changed the adjusted R(2) for the combination very little. Between scale levels 1A and 5B mean (SD) VA decreased from 86.8 (5.8) letters to 59.8 (13.6) letters. In a model of change in VA as a function of time spent at each DME severity level, VA loss increased progressively from 1 letter per year at level 2 to 17 letters per year at level 5B. CONCLUSIONS: The scale facilitates documentation of the relationship of severity and duration of DME with VA.