A clinico-anatomical evaluation of the evidence on 3436 autopsies (1091 cases of cancer included) was carried out. The levels of glucagon-producing A- and insulin-producing B-cells were studied in pancreatic islands from 148 autopsied cases (33 cases of cancer included). Diabetes mellitus incidence in non-tumor cases was thrice (13.73%) that in cancer patients older than 54 years (4.15%). However, pancreatic islands' A-cell levels were higher than those of B-cells in both groups, such prevalence being generally regarded as a causative factor of diabetes mellitus development. The study was concerned with incidence of diabetes mellitus in cases of different primary cancers, the significance of cachexia, corticosteroid production by cancers of the adrenal cortex and some other factors which influence clinical manifestations of diabetes mellitus, particularly, those related to age-linked changes in A/B-cells ratios in pancreatic islands.