Clinical factors related to the development and progression of renal lesions were studied in twenty-three diabetics by the use of serial renal biopsies or autopsy. The results were as follows: Most of the juvenile and intermediate type diabetics were poorly controlled, with the glomerular lesion progressing rather rapidly. In contrast, many cases of the adult type were able to be maintained under good control and the renal lesion neither developed nor progressed. Two of the adult type diabetics with poor control showed slowly and slightly progressing renal lesions. The progression of glomerular lesions was significantly related to the control of blood glucose, type of diabetes, age at onset, type of treatment, and degree of obesity, but not to the duration of diabetes or the length of the follow-up period. There was a significant correlation between the type of diabetes and the control of blood glucose over the years. Arteriolar lesions developed concurrently with the progression of the glomerular lesion. Retinopathy also had a tendency to develop in proportion to the progress of glomerular lesions although it was not statistically significant. We have discussed the clinical factors responsible for the progression of diabetic glomerulosclerosis and have suggested that the type of diabetes rather than the degree of control of blood glucose might be more important in determining the development and progression of diabetic glomerulosclerosis. Nevertheless, the possibility remains that successful control of blood glucose may prevent or retard the development of diabetic glomerulosclerosis.