BACKGROUND and AIMS: There is still some uncertainty towards the best method to diagnose the sarcopenic obesity, and specifically, altered body composition. A recent international consensus proposes the association of increased fat mass (FM) with reduced skeletal muscle mass adjusted by weight (SMM/W) (method 1). The ratio of fat mass index (FMI)/fat free mass index (FFMI) (method 2) has also been proposed. AIMS: to determine and compare the prevalence of altered body composition by the two methods, and its associated factors, in obese patients referred to a specialized center. METHODS: Anthropometry, single-frequency bioimpedance analysis (BIA), and risk factors for sarcopenia were collected. SMM, according to the Janssen equation, and the FMI/FFMI ratio, were calculated. The agreement between the two methods was determined by the kappa coefficient. Multivariable analysis identified the factors associated with altered body composition according to the two methods. RESULTS: 253 consecutive patients were retrospectively included: 81 % women, age 44.4 ± 13 yr, body mass index (BMI) 43.0 ± 6.8 kg/m(2), severe obesity, 94 %. Altered body composition was reported in 90 % of the obese patients (n = 227/253) with method 1, and 35 % (83/253) with method 2. The agreement between the two methods was very low (kappa = 0.10). The factors associated with altered body composition were: method 1, age (odds ratio = 1.05 [95 % confidence interval, 1.01; 1.10], P = 0.02), arterial hypertension (0.18 [0.06; 0.57], P = 0.04), hip circumference (1.13 [1.07; 1.20], P < 0.0001), previous obesity follow-up (3.78 [1.30; 10.96], P = 0.002); method 2, female gender (3.12 [1.02; 9.5], P = 0.04), age (1.10 [1.06; 1.14], P < 0.0001), BMI (1.37 [1.26; 1.50], P < 0.0001), history of cancer (5.35 [1.20; 23.84], P < 0.028), phase angle (0.42 [0.27; 0.66], P < 0.001). CONCLUSION: Altered body composition is common in severely obese patients. The two methods are not comparable. Further work will aim at refining the methods of body composition measurements in the obese patients, and especially for the sarcopenic obesity diagnosis.