A model of intermediate inheritance of diabetes mellitus is given, according to which juvenile diabetes developes in persons homozygous for the mutant gene, and a part of heterozygous carrier of the mutant gene has late diabetes mellitus. A large group of diabetes cases (about 50%), which are not due to direct diabetes heredity, is taken into account. Calculations are carried out on the basis of Hardy-Weinberg law from the patient questionaire data. The frequency of juvenile diabetes (developing within first thirty years) is 0,028% in the Leningrad population, the frequency of heterozygous diabetes gene carrier being 3,28%. Penetrancy of the diabetes gene in heterozygous state, as estimated from registrable (clinically expressed) forms of the disease, is 8,4%. The risk of juvenile diabetes in children, if one of the parents has juvenile diabetes, is 1,65% (3,3%-1/2), the frequency of heterozygous carriers in the population being 3,3%. The analysis of questionaire data of diabetes patients produced results, which were close to expected and indirectly confirmed the accurancy of the model proposed.