1 In 4058 diabetics, an inverse relation (negative semilogarithmic correlation) was found between the age of onset and the familial aggregation of diabetes mellitus (P less than 0.01). Hence, juvenile (especially infantile) diabetes appears to be predominantly caused by genetic defects (mutations), whereas adult onset diabetes appears to be essentially based on epigenetic factors. 2. In 3890 diabetics with an onset age of more than 10 years, a highly significant predominance of familial diabetes aggregation was observed on the maternal side as compared to the paternal side (P less than 0.001). This finding suggests the possibility of an epigenetic ("teratogenetic") mode of diabetes transmission mediated by the mother.