Diabetic macular edema is a major cause of vision loss and is evaluated with qualitative or semiquantitative techniques. A new quantitative method for assessment of macular edema using retinal thickness analysis was applied to 19 patients with diabetic macular edema. Foveal thickening was frequently coupled with poor visual acuity. Slit-lamp biomicroscopy and stereophotography detected 80% and 78% of local areas of thickening, respectively, but failed to detect locations with average thicknesses of 1.5 and 1.6 times normal, respectively. Fluorescein leakage on angiography was generally associated with retinal thickening, but locations with similar degrees of leakage had widely varying retinal thickening. Fluorescein leakage in the posterior vitreous correlated poorly with the degree of foveal thickening. These results indicate that quantitative measurement of retinal thickness may become useful in the management of diabetic patients with macular edema.