Joint capsular tissue from both knee joints have been investigated light microscopically and morphometrically in 40 cases with maturity onset of diabetes. The tissue specimen were taken from autopsy cases (27 female, 13 male). The following alterations of synovial tissue can be regarded as typically in diabetes mellitus: 1. An increased formation of synovial villi without proliferation of synoviocytes and without inflammatory reaction. 2. Increased hyalinosis and fibrosis of the synovial villi as well as fibrosis of the flat synovial membrane. The alterations were far developed in more than 50% of all cases with diabetes.