Sixteen insulin dependent diabetic patients (age at onset less than 35 years) and their families were tissue typed for HLA antigens. Glucose tolerance of relatives was also tested. Among diabetic patients two HLA antigens were found with increased frequency: B8 (31 percent, control 15 percent) and Bw35 (38 percent, control 23 percent). Among normal relatives B8 and Bw35 had the same frequency as the control group. Bw15 frequency was not increased in either group. In relatives, no correlation between HLA antigens (B8 or Bw35) and abnormal glucose tolerance, obesity and over-weight at birth was found. Present data confirm previous reports of high B8 frequency in early onset diabetic patients, but fail to demonstrate a raised frequency of abnormal glucose tolerance among relatives bearing B8 (or, in our cases, Bw35). B8 may be considered a genetic indicator for susceptibility to juvenile diabetes. On the basis of present results in families, however non genetic factors clearly also play a determinant role. Furthermore, that diabetogenesis arises from a link between Ir-genes and HLA-B8 antigen should only be considered a suggestive hypothesis.