Malnutrition has a significant impact on the recovery of patients. Assessment of nutritional status and appropriateness of nutritional support is of clinical importance. In the various nutritional assessment methods, biochemical markers (albumin, pre-albumin, retinol binding protein, and transferrin) are widely used for high sensitivity and objectivity. For application of the biochemical markers, it should be understood that the markers have merits and de-merits. Author investigates the retinol binding protein, one of the most sensitive biochemical markers, in more detail. Retinol binding protein (RBP) is synthesized in liver (mainly, parenchymal cells) and catalyzed in kidney. RBP transports retinol, alcohol form of vitamin A, from liver to tissue. Also, RBP and transthyretin (TTR, formerly called pre-albumin) form a macromolecular complex to prevent glomerular filtration of the low molecular weight RBP in the kidney. RBP is a very useful biochemical marker because it has short half-life and immediate response to deficiency or in support of calorie and protein. However, because serum RBP level is greatly affected by the liver and kidney function, understanding of the underlying disease of patients is necessary. Moreover, it is not widely used due to very short half-life and non-generalized measurement methods. Consequently, understanding the characteristics of RBP is necessary and effort should be made to properly utilize the RBP in nutrition support and assessment.