Recently the spontaneous development of insulin autoantibodies (IAA) has been detected in patients at diagnosis of Type I diabetes mellitus before the beginning of insulin treatment. The present study was undertaken to investigate if the presence of IAA at clinical onset of IDDM may act as a new marker of the beta cell function. The results obtained showed that IAA were present in 44% of newly diagnosed diabetic patients before therapy. Patients without IAA displayed a higher C-peptide secretion than those with IAA, at six months (12.11 +/- 5.08 versus 5.88 +/- 3.25 ng/ml/10 min.)(X +/- SD) and at twelve months (10.45 +/- 3.05 versus 4.90 +/- 5.25 ng/ml/10 min)(X +/- SD) of the follow up period. HbA1 levels, and insulin requirements were similar in both groups (IAA+ and IAA-). We conclude that the presence of insulin autoantibodies at clinical diagnosis, before initiating insulin treatment, may well predict the loss of the beta cell function.