The risk factors for the occurrence and progression of diabetic retinopathy were estimated in 1084 diabetic subjects (4358 person-years) by logistic regression analysis. The diabetic duration, the average of HbA1C for every year of observation, the range of HbA1C, the difference of HbA1C between the first visit to our clinic and the average of one year of HbA1C were statistically significant covariates for the risk of the occurrence of retinopathy. The relative risks of the occurrence of retinopathy as compared with the diet were estimated as 2.02 in oral administration of medicine and 1.86 in insulin therapy. The interaction between the difference of HbA1C and the average of HbA1C was statistically significant for the progression of retinopathy. When the response surface was estimated, the risk of progression of retinopathy differed greatly according to the blood glucose control level.