The effect of the start of glycemic control on retinopathy progression was investigated. The changes of glycosylated hemoglobin (HbA1) were compared between the retinopathy progression group (Group 1, 24 eyes) and the non-progression group (Group 2, 23 eyes). The follow-up period was 24 months. The grades of retinopathy, age, duration of diabetes mellitus (DM), sex, and manner of DM control in these subjects were matched between the two groups. They were classified into 9 strata according to the matched systemic factors. In Group 1, HbA1 value decreased rapidly 6 or 7 months before the retinopathy progression, but did not change in Group 2 during 24 months. The change rate was significantly different between Group 1 and Group 2. The block efficacy for 6-7 months follow-up was also statistically significant between the two groups by analysis of variance (ANDVA). The estimated mean HbA1 value for the 9-10 month period in Group 1 was 2.46% less than that of Group 2. The higher the HbA1 value, the higher the relative risk (odds ratio) of retinopathy progression. The relative risk of retinopathy progression of the mean HbA1 value was highest for 9-10 month period. The relative risks of 1, 2, and 3% decrease of HbA1 for 9-10 months were 1.7, 2.8, and 4.7, respectively.