Retinol-binding protein 4 (RBP4; formerly called RBP) is a plasma transport protein for vitamin A (retinol). Synthesized primarily by hepatocytes and adipocytes, RBP4 is secreted into the circulation as a retinol-RBP complex and functions to transport retinol to peripheral tissues. In plasma the RBP4-retinol complex is bound to transthyretin (TRR) to prevent kidney filtration. The C-terminally processed forms of RBP4, which do not bind TTR, are normally excreted into the urine but accumulate in the serum during renal failure. Urinary RBP4 has been reported as marker for glomerular disease. RBP4 also was identified as an adipokine that elevated in some insulin-resistant states. Measurement of serum RBP4 could be used to assess the risk of insulin resistance, type 2 diabetes, obesity, and cardiovascular disease.