PURPOSE: The purpose of this study was to investigate the correlation between best-corrected visual acuity and foveal microstructural changes of the external limiting membrane and the junction between the inner and outer segments of the photoreceptors in diabetic macular edema. METHODS: The authors performed spectral-domain optical coherence tomography in all eyes. The authors defined central subfield thickness as the average retinal thickness of the 1-mm central scanned area. The length of disruption of the external limiting membrane and the inner and outer segments of the photoreceptors in the fovea (1.8 mm in diameter) were measured and graded according to their length as follows: 1) >1.4 mm; 2) >0.4 mm but <1.4 mm; or 3) <0.4 mm. RESULTS: The best-corrected visual acuity was strongly associated with the external limiting membrane (r = 0.66) and inner and outer segments of the photoreceptor (r = 0.68) scores, whereas best-corrected visual acuity was weakly and negatively correlated with central subfield thickness (r = -0.23). CONCLUSION: Spectral-domain optical coherence tomography showed that the integrity of the external limiting membrane and inner and outer segments of the photoreceptors was more strongly correlated with best-corrected visual acuity when compared with central subfield thickness in diabetic macular edema.