Data on the protein requirements of elderly adults are limited, because it is impractical to conduct repeated nitrogen balance protocols in these vulnerable humans. This study was designed to determine the dietary protein requirement of elderly women by using the recently developed minimally invasive indicator amino acid oxidation (IAAO) technique. Six white women aged 80–87 y mean ± SEM: 82 ± 1 y and body mass index (in kg/m2) 26 ± 2 completed a 3-d protocol 7 times. Each woman consumed an adaptation diet for 2 d and on day 3 consumed a complete test diet with a crystalline amino acid mixture containing 1 of 7 protein intakes (0.1, 0.3, 0.6, 0.9, 1.2, 1.5, or 1.8 g · kg−1 · d−1) tested randomly. A group-based protein requirement was assessed by using a nonlinear mixed model of protein intake and l-1-13Cphenylalanine oxidation. The breakpoint, at which there was no further decline in the rate of appearance of 13C in the breath, was used as an index of the mean protein requirement. The mean protein requirement (95% CI) was 0.85 (0.60, 1.09) g · kg−1 · d−1. This requirement is 29% higher than the current Estimated Average Requirement (EAR) for adults of 0.66 g · kg−1 · d−1 based on the nitrogen balance technique, although the 95% CI includes the current EAR. The corresponding adequate protein allowance of 1.15 (0.77, 1.54) g · kg−1 · d−1 is 44% higher, although the 95% CI includes the Recommended Dietary Allowance (RDA) of 0.80 g · kg−1 · d−1. Notwithstanding uncertainty about the validity of the use of the IAAO technique to assess protein requirements, the results of this study with octogenarian women suggest that the current EAR and RDA for elderly women may be underestimated. The limitations of this short-term, noninvasive method underscore the need for new research that uses alternative experimental designs and measuring physiologic, morphologic, and health-related outcomes. This trial was registered at clinicaltrials.gov as NCT01193946.