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The purpose of this study was to determine a cut-off value of the molar ratio of retinol-binding protein to transthyretin (RBP:TTR) to indicate marginal vitamin A (VA) deficiency. Plasma RBP and TTR were measured by radial immunodiffusion in two groups of patients, i.e., surgical patients with known hepatic VA stores, and a cohort of children residing in a malaria-endemic area of Papua New Guinea who had received placebo or 210 [micro]mol of VA every 3 mo for 9 mo. A RBP:TTR ratio [less than or equal to] 0.36 selectively detected five of seven patients (71% sensitivity) with hepatic VA stores [less than or equal to] 69.9 nmol/g of tissue (i.e., [less than or equal to] 20 [micro]g/g), indicative of marginal VA deficiency. Using this cut-off value, 28% (n = 245) of children from Papua New Guinea had marginal VA deficiency before supplementation. After 7 mo, a low ratio persisted in 29% (n = 92) of placebo-treated children but in only 11% (n = 83) of those receiving VA supplements ([chi square], P < 0.01). At the end of the study, 13 mo after initiation or 4 mo after the last dose of VA, the percentage of children with a low ratio was still lower ([chi square], P < 0.02) in the VA group, 42.5% (n = 113) than in the placebo group, 58.6% (n = 118). These results demonstrate that a cut-off value [less than or equal to] 0.36 is indicative of marginal VA deficiency and can be used as an indirect method of VA assessment. J. Nutr. 132: 3687-3692, 2002. KEY WORDS: * humans * liver * malaria * vitamin A status