Cultured skin fibroblasts from clinically normal offspring of two parents with non-insulin-dependent diabetes have demonstrated premature senescence as a decreased ability of cells to establish colonies when inoculated at low density (plating efficiency). The present study tested the hypothesis that there is an inherent cellular defect affecting viability of diabetic cells in insulin-dependent diabetes. Four insulin-dependent patients, aged 12 to 19 years, included two with joint contracture, skin changes, and growth failure; one with thyroiditis and past history of nephrosis; and one with a family history of insulin dependency. Ten control subjects, aged 10 to 52 years, had negative family histories and normal oral glucose tolerance tests. Number of cells per confluent dish correlated significantly with donor age (p less than 0.001) at 30 and 40 in-vitro generations. The patients' cells' mean confluent density did not differ from that of five age-matched controls. Plating efficiency correlated with donor age at 30 in-vitro generations ( p less than 0.001); plating efficiency of cells from the youngsters with diabetes was virtually identical to that of control cells at 20, 30, and 40 generations. In this small series of two subjects with in-vivo growth failure, one with associated autoimmune disease and another with familial insulin-dependent disease, cultured fibroblasts demonstrated normal viability and the hypothesis of a cellular growth defect was not confirmed.