Clinical and experimental studies have delineated a link between dietary cow milk protein and the development of insulin-dependent diabetes mellitus (IDDM), and bovine serum albumin (BSA) was proposed as one candidate mediator of this effect. The demonstration of anti-BSA antibodies in new onset type 1-diabetic children from Finland initiated a controversial debate on the utility of BSA antibodies as a disease marker and on the role of BSA in IDDM. Here we analyzed BSA antibodies in newly diagnosed type 1-diabetic patients and their first degree relatives, patients with other autoimmune diseases, and children with Down's syndrome from Germany. Blinded serum samples (n = 308) were screened for IgG anti-BSA antibodies by particle concentration fluoroimmunoassay (PCFIA). The prevalence of elevated BSA antibodies in newly diagnosed type 1-diabetic patients was low (11%), although mean BSA antibody levels were significantly increased in diabetic patients as compared to controls (1.94 +/- 1.51 vs. 0.97 +/- 0.93 kFU, p < 0.0007). Mean BSA antibody levels were also increased in ICA+ and/or IAA+ first degree relatives (1.32 +/- 0.43, p < 0.002) and in children with Down's syndrome (3.01 +/- 1.93, p < 0.0007), but not in the other autoimmune disorders tested. The low prevalence of elevated anti-BSA levels in IDDM patients limits the clinical usefulness of this immune marker. We conclude that current anti-BSA assays do not substantially contribute to the prediction and diagnosis of IDDM.