Background The prospective DIabetes and CATaract Study II (DICAT II) was performed to characterise the risks of cataract surgery to the retinae of patients with early diabetic macular oedema (E-DMO). Methods DICAT II was a prospective, comparative, multicentre, observational study involving six Italian clinics. Patients were aged >= 55 years, had type 1 or 2 diabetes with spectral-domain optical coherence tomography evidence of ESASO classification Early DMO. Group 1 eyes (78 eyes, 78 patients) underwent phacoemulsification-based cataract surgery. Group 2 eyes (65 eyes, 65 patients) had E-DMO and either clear media or had undergone uncomplicated cataract surgery >= 1 year previously. Central subfield thickness (CST) and best-corrected visual acuity (BCVA) were assessed in both groups. Results The negative impact of surgery on CST was evident after the first postoperative week; CST peaked during the first month, then rapidly decreased. CST worsening >= 10 mu m was observed in 63/78 eyes (80.7%) and 29/65 eyes (44.6%) in Groups 1 and 2, respectively (p < 0.0001). CST worsening of >= 50 mu m was observed in 51 eyes (65.4%) and 10 eyes (15.4%) in Groups 1 and 2, respectively (p < 0.0001). Mean CST worsening was lower in Group 2 than in Group 1 (38.6 +/- 30.4 mu m vs 85.5 +/- 55.3 mu m, p < 0.0001) with a lower BCVA loss (-2.6 +/- 3.5 letters vs -8.2 +/- 6.2 letters, p < 0.0001). Higher glycaemic levels and HBA1c levels were significantly associated with the risk of >50 mu m CST worsening in eyes from both groups. Conclusion Early DMO is associated with poorer outcomes after cataract surgery and requires close pre- and postoperative monitoring.