Purpose: To determine whether the extensiveness of diabetic macular edema using a 10-step scale based on optical coherence tomography explains pretreatment variation in visual acuity and predicts change in macular thickness or visual acuity after laser photocoagulation. METHODS: Three hundred twenty-three eyes from a randomized clinical trial of two methods of laser photocoagulation for diabetic macular edema were studied. Baseline number of thickened optical coherence tomography subfields was used to characterize diabetic macular edema on a 10-step scale from 0 to 9. Associations were explored between baseline number of thickened subfields and baseline fundus photographic variables, visual acuity, central subfield mean thickness (CSMT), and total macular volume. Associations were also examined between baseline number of thickened subfields and changes in visual acuity, CSMT, and total macular volume at 3.5 and 12 months after laser photocoagulation. RESULTS: For baseline visual acuity, the number of thickened subfields explained no more variation than did CSMT, age and fluorescein leakage. A greater number of thickened subfields was associated with a greater baseline CSMT, total macular volume, area of retinal thickening, and degree of thickening at the center of the macula (r = 0.64, 0.77, 0.61-0.63, and 0.45, respectively) and with a lower baseline visual acuity (r = 0.38). Baseline number of thickened subfields showed no association with change in visual acuity (r ≤ 0.01-0.08) and weak associations with change in CSMT and total macular volume (r from 0.11 to 0.35). CONCLUSION: This optical coherence tomography based assessment of the extensiveness of diabetic macular edema did not explain additional variation in baseline visual acuity above that explained by other known important variables nor predict changes in macular thickness or visual acuity after laser photocoagulation.