BACKGROUND: The decline in lean mass, observed in older people, has been frequently associated with frailty. This assumption has scarcely been assessed. This study explored the association between current proposed definitions of low lean mass and the Fried phenotype of frailty. METHODS: Cross-sectional study. Participants admitted to the Toulouse frailty day hospital, with an assessment of body composition, 70 years or older were included consecutively in the study. Low lean mass (LLM), was identified using five international operative definitions. To construct the definitions, muscle mass was assessed using Intelligent Dual Energy X-ray absorptiometry (I-DXA, LUNAR). Frailty was assessed using the Fried criteria. RESULTS: Data from 283 participants, mean age 82 years and 71% of women were analyzed. LLM was identified between 8.5% and 39.2% of the participants according to the different definitions. Frailty was identified in 46.6% of the sample. 9.1%-48.5% of the frail older people had LLM depending on the definition. The highest association between frailty and LLM was observed with the definition proposed by the Foundation for the National Institutes of Health (FNIH) Sarcopenia Project [adjusted Odds Ratio 2.64; 95% confidence interval 1.5-4.8]. CONCLUSION: The decline in lean mass is a component of the frailty syndrome but not universally present. Indeed, LLM and frailty were associated and partly overlapped. Future research including longitudinal studies should exploit the added value of combining LLM and frailty measures in preventing disability and other negative health outcomes.< Réduire