AIM: To assess quantitative changes of the macula in diabetic eyes after cataract surgery using optical coherence tomography (OCT) and to estimate the incidence of development or worsening of macular edema (ME) in diabetic eyes with or without pre-existing ME. METHODS: In this prospective, observational study, 92 eyes of 60 diabetic patients who underwent cataract surgery were evaluated before surgery and 1, 3mo after surgery using OCT. Macular thickness was measured with OCT at nine macular subfields defined by the 9 zones early treatment of diabetic retinopathy study (ETDRS), as well as total macular volume obtained by OCT at 1, 3mo after surgery were compared with baseline features obtained before surgery. In addition, the incidence of development or worsening of ME was analyzed in diabetic eyes with or without pre-existing ME. RESULTS: The central subfield mean thickness increased 21.0 μm and 25.5 μm at 1, 3mo follow-up, respectively (P<0.01). The average thickness of inner ring and outer ring increased 14.2 μm and 9.5 μm at 1mo, 18.2 μm and 12.9 μm at 3mo. Central-involved ME developed in 12 eyes at 3mo, including 4 eyes with pre-existing central-involved and 8 eyes with pre-existing non-central involved ME. Pre-existing diabetic macular edema (DME) was significantly associated with central-involved ME development (P<0.001). CONCLUSION: A statistically significant increase could be detected in the central subfield as well as perifoveal and parafoveal sectors though the increase was mild. And eyes with pre-operative DME prior to cataract surgery are at higher risk for developing central-involved ME.