OBJECTIVE To evaluate the prevalence of anti-bovine serum albumin (BSA) antibodies in patients with adult-onset insulin-dependent diabetes mellitus (IDDM) and investigate a possible link between their presence and genetic susceptibility or resistance determined by human leukocyte antigen (HLA) complex. METHODS Sera from 60 recent-onset diabetic patients, 5 prediabetic subjects, and 102 healthy control subjects were tested using a radioimmunoprecipitation assay. HLA-DRB and -DQB alleles were determined by means of allele-specific oligonucleotide typing. Islet cell antibodies (ICAs) were assayed by indirect immunofluorescence. RESULTS Levels of anti-BSA antibodies were significantly higher in IDDM patients (18.1 +/- 3.5%, n = 60) than in healthy control subjects (7.5 +/- 1.2%, n = 102) (P < 0.001), but in only 16.6% of IDDM patients (10 of 60) were the titers above the 95th percentile of control values. Anti-BSA antibody titers were higher in HLA-DR3 and/or -DR4 patients (23.4 +/- 4.9%, n = 41) compared with DR3 and/or DR4 control subjects (3.1 +/- 1.0%, n = 10) (P < 0.001). DR3 IDDM patients showed higher levels of anti-BSA antibodies (26.3 +/- 6.3%, n = 30) than non-DR3 patients (9.9 +/- 2.6%, n = 30) (P < 0.01) and healthy control subjects. Only two out of five prediabetic subjects had significant anti-BSA levels before clinical onset of diabetes. CONCLUSIONS Our data confirm that antibodies to BSA are present in adult-onset IDDM patients, particularly in HLA-DR3-positive patients. However, the prevalence of anti-BSA antibodies was lower than previously reported in children, and there was a considerable overlap with healthy control subjects. Only two out of the five prediabetic patients demonstrated anti-BSA antibodies. Taken together, these results do not bring strong support to the clinical usefulness of anti-BSA antibodies as a relevant marker in diabetes prediction or diagnosis.