Vitamin A is an essential nutrient that is obtained from the daily diet. The major forms of vitamin A in the body consist of retinol, retinal, retinoic acid (RA), and retinyl esters. Retinal is fundamental for vision and RA is used in clinical therapy of human acute promyelocytic leukemia. The actions of retinol and retinyl palmitate (RP) are not known well. Recently, we found that retinol is a potent anti-proliferative agent against human refractory cancers, including gallbladder cancer, being more effective than RA, while RP was inactive. In the current study, we determined serum retinol concentrations in xenograft mice bearing tumors derived from four refractory cancer cell lines. We also examined the effects of vitamin A on proliferation of human gallbladder cancer cells in vivo. Serum retinol concentrations were significantly lower in xenograft mice with tumors derived from various refractory cancer cell lines as compared with control mice. The growth of tumors was inhibited with increasing serum retinol concentrations obtained post-administration of RP. In addition, pre-administration of RP increased serum retinol concentrations and suppressed tumor growth. These results indicate that administration of RP can maintain retinol concentrations in the body and that this might suppress cancer cell growth and attachment. The regulation of vitamin A concentration in the body, which is critical biomarker of health, could be beneficial for cancer prevention and therapy.