Present knowledge regarding the HLA system and the association between HLA antigens and insulin-dependent type 1 diabetes mellitus (IDDM) is reviewed. The heterogeneity of diabetes, immunogenetically speaking, is emphasized. Results are reported for HLA typing in 18 cases of known IDDM recently diagnosed and observed at the Karen Bruni Diabetes Center in approximately one year (1981-82). The frequency of HLA antigens B7, B8 (in linkage disequilibrium with DR3), B15 (in linkage disequilibrium with DR4) and B18 was examined in comparison with a Piemontese control group. The X2 method was used for calculating the relative risk and statistic importance of the intensity of association. IDDM susceptibility in association with HLA-B18 was confirmed and resulted significantly higher in our cases in respect to controls. Correlations without, however, reliable importance, have also been found between HLA-B8 and B15. IDDM protection by HLA-B7 was not confirmed. Diabetes began during the winter, from October to February, in 10 out of 18 cases, and some were positively related to a previous respiratory viral infection. Previous virus infection was found in three B7-positive cases. The more frequent arousal of diabetic symptoms during the winter in subjects positive for HLA-B8 and B18 was confirmed in 7 out of 8 cases. This work demonstrates the current practicability of HLA typing of recently diagnosed insulin-dependent diabetic in a Diabetes Center. This element helps to a more correct classification--on a subclinical basis--of initial cases of type 1 and 2 diabetes and can be used for possible problems during the course of insulin therapy.