In 30 diabetics and their nearest relations (44 persons) the immunoreactive insulin (IRI) was examined and a routine glucose tolerance test was carried out as well as in patients with ischaemic heart disease (more than 100 patients). The control group consisted of 30 healthy volunteers. An analysis of the results was performed taking into consideration the degrees of severity and the duration of the diabetes as well as the age of the patients. A conclusion was drawn that the exhaustion of Langerhans islets occurs relatively early during "juvenile" diabetes. A clear decrease of the IRI corresponding to the degree of severity and to the duration of diabetes can be observed. In the nearest relations of the diabetics a normal IRI-level and in a relatively little percentage latent forms of a diabetes were detected. In patients with ischaemic heart disease however, the non-manifest forms of diabetes were by far more frequent. It is considered that more precise tests should be applied in laboratory diagnosis of latent diabetes mellitus in hereditarily aggravated persons in the mentioned respect.