The purpose of this study was to investigate through an epidemiological approach two controversial aspects of the pathogenesis of the diabetes mellitus of idiopathic haemochromatosis (I.H.) : the possible inheritance of the gene(s) for common diabetes mellitus (C.D.), and the diabetogenic role of iron overload. More than 80% of the living first degree relatives of 97 patients with I.H. were examined, while data were collected by inquiry concerning first degree relatives who had refused investigations or had died. Data on the more distant family members were also collected by inquiry. Physical examination and estimation of serum iron level and unsaturated-iron-binding capacity were systematically performed. When an anomaly had been thus detected further investigation for iron overload was carried out by mean of a deferoxamine test and eventually by liver biopsy. Evaluation of carbohydrate metabolism included testing for post-prandial glycosuria, estimation of post-prandial blood sugar, and eventually an oral glucose tolerance test. The results were compared to those of an inquiry for family history of diabetes in 100 patients with C.D. successively admitted to our department. Among the first degree relatives of patients with C.D. the prevalence of overt diabetes was 33 of 612 (5.4 %); whereas in the I.H. group it was 8 of 735 (1.1 %). The differences between the C.D. and I.H. groups were significant, whether the total I.H. group (p less than 10(-5)) or only I.H. proposite having overt diabetes (p less than 2 X 10(-2)) were considered. With respect to the more distant relatives the number of affected families was significantly higher in the C.D. group (31 of 100) than in the total I.H. group (5 of 97 ; p less than 10(-5)) or in the I.H. sub-group diabetic proposite (3 of 36 ; p less than 10(-2)). The frequency of carbohydrate intolerance in relatives bore no relation to the carbohydrate pattern of propositi. Carbohydrate intolerance was frequently found in relatives with iron overload (17 of 72). However, no correlation was observed between blood sugar and serum iron level or unsaturated-iron-binding-capacity, relatively gross parameters. Thus, the pathogenesis of diabetes mellitus associated with I.H. remains uncertain, but the inheritance of gene(s) for common diabetes is unlikely to play a determinant role.