Aim: To prospectively evaluate the efficacy of subtenon injection of triamcinolone acetonide (TA) before laser grid pattern photocoagulation (G-PC) for the treatment of diffuse diabetic macular oedema (DDME). Methods: 42 eyes of 37 consecutive patients with DDME were studied. 1 week before G-PC, 21 eyes received TA subtenon injection, and the other eyes served as control. The clinical course of visual acuity (VA) and foveal thickness (FT) was monitored for up to 24 weeks after G-PC. Mean deviation (MD) of perimetry with 30-2 program on Humphrey Perimeter (Zeiss-Humphrey, Dublin, California, USA) was also measured. The average laser intensity was recorded. Results: After TA injection, FT and VA were improved, and subsequent G-PC maintained the improvement for up to 24 weeks without recurrence of diffuse diabetic macular oedema. In contrast, G-PC without TA injection induced transient worsening of FT and VA, then both were gradually improved. At 24 weeks after G-PC, MD in the TA-injected eyes was better than those in control. The required laser intensity in TA-injected eyes was less than that for control. Conclusion: Subtenon injection of TA prior to G-PC allows for treatment with a lower intensity of laser spots and also prevents the decrease in central visual field sensitivity, all of which have clinical advantages for G-PC.