An albumin dimer (approximately 134 000 Da) was present, along with monomeric albumin, in freshly voided urine but not in serum from a 53-year-old man with alcoholic liver disease and chronic renal failure. The dimerization, evidently via disulfide bonds, resulted in a loss of [125I]thyroxin-binding capacity. This suggests that the S--S bridging is at a site different from that previously reported. The dimer was unstable at all storage temperatures studied.