The effect of 100% oxygen breathing on retinal blood flow was investigated using laser Doppler velocimetry in 19 normal eyes, and in 41 eyes of insulin treated diabetic patients. Of the diabetic eyes studied, nine had no retinopathy, 18 had background diabetic retinopathy, seven had proliferative diabetic retinopathy, and seven had proliferative diabetic retinopathy that had been previously treated by argon panretinal photocoagulation. Five minutes of 100% oxygen breathing produced an average decrease in blood flow of 61% (SD = 8) in normal eyes, 53% (SD = 10) in NR eyes, 38% (SD = 13) in background diabetic retinopathy eyes, 24% (SD = 18) in proliferative diabetic retinopathy eyes and 54% (SD = 8) in panretinal photocoagulation eyes. In six eyes with proliferative retinopathy measured before and after panretinal photocoagulation, a significant increase in vascular response to O2 was observed following photocoagulation (Wilcoxon signed rank test, P less than 0.05).