We studied attributes of diabetic sib pairs in 138 insulin-dependent multiplex families. Within multiply affected families, there is a large subset of siblings diagnosed only a year or less apart in whom the usual seasonal variation for diabetes is reversed. Furthermore, we confirm that affected sibs tend to cluster for onset of disease. The clustering, however, is limited to those sibs younger than 20 yr. These diabetic patients cannot be categorized by HLA type, and a common environment may play a major role in the development of insulin-dependent diabetes.

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