This study is based on a material consisting of all first-degree relatives and a group of second-degree relatives (all nieces and nephews) of 187 propositi affected by early onset (i.e. at or prior to 20 years of age) diabetes mellitus, diagnosed before 1946. The incidence of diabetes among the different groups of relatives is expressed as the risk of developing the disease at specified ages. It is concluded that the risk of siblings or children of early-onset diabetics developing the same disease is about ten times that of a normal population chosen for comparison, whereas the risk of siblings developing diabetes later in life does not differ from this normal population. On the basis of the data presented here, combined with reanalyses of previously published data from other authors, it is concluded that early onset and late onset diabetes cannot have an identical genetic background.