To study the evolution of the bone mass by ultrasonic transmission after biliopancreatic diversion. Forty eight morbid obese patients were prospectively studied during 36 months following the Larrad biliopancreatic diversion. The bone metabolism was studied by PTHi and the urinary pirydinolines. The bone mass by echography and bone densitometry, which correlate to the levels of PTHi and pyridinolines. After 3 years the bone mass decreased from 50.15 +/- 7.31 kg/m(2), preoperatively, to 34.03 +/- 4.53 kg/m2 (p < 0.001). There was an increase of the PTHi value (from 71.4 +/- 79.6 to 91.65 +/- 43.06 pg/ml) (p = 0.01), and the urinary pirydinolines (from 7.93 +/- 4.06 an 11.4 +/- 10.12 nM/mM) (p < 0.05). The ultrasonic transmission speed increased (from 1,990.93 +/- 62.38 to 2,035.25 +/- 53.98 m/s). However, the bone mineral content (BMC) did not show changes (from 3,016.5 +/- 562.8 to 2,909.6 +/- 304.2 g), as well as the Bone Mineral Density (BMD) (of 1,174.2 +/- 98.8 g/cm2). Neither correlation was found between the BMD (r = 0.212; p = 0.6), the BMC (r =-0.125; p = 0.768), and the T-score (r = 0.592, p = 0.093). The study of the bone mass through ultrasonic transmission speed revealed low sensitivity during the assessment of the morbid obese patients. A percentage of cases of osteopenia were observed despite the fact that they are not reflected in the bone content or in the bone mass. Ultrasonic evaluation of bone mass has no value in the morbidly obese, by the clear negative correlation between ultrasound velocity and thickness of soft tissue.