Purpose: To compare the safety and efficacy of intravitreal (IVT) and sub-Tenon (ST) triamcinolone acetonide for the treatment of refractory diffuse diabetic macular oedema. Method: 29 eyes of 22 patients with long-standing, diffuse diabetic macular oedema refractory to argon laser treatment were randomly assigned to a single 4mg injection of IVT triamcinolone acetonide or a 40mg sub-Tenon injection. Patients were subsequently monitored for six to nine months. Outcome measures were visual acuity, intraocular pressure, macular thickness on optical coherence tomography and adverse effects. Results: There was no significant improvement in visual acuity in either group. A transient decrease in macular thickness was found in the IVT group but not in the ST group. There were no significant adverse effects apart from a mild to moderate intra-ocular pressure rise found more frequently in the IVT group. Conclusion: IVT and ST triamcinolone acetonide injections for refractory diffuse diabetic macular oedema appear relatively safe and well-tolerated. IVT injection produces a significant temporary decrease in macular thickness in patients with long-standing diffuse diabetic macular oedema while ST injection does not. Neither intervention was shown to significantly improve visual acuity in this group of patients.