Very few studies have examined the effect of childhood obesity on bone mineralization. The purpose of the present study was to determine bone strength by quantitative ultrasound (QUS) measurements of bone speed of sound (SOS) in obese children (n = 45; age range 6-17 yr). SOS was measured (by Sunlight Omnisense) in the mid-tibial and radial bones. Data were compared to age- and gender-matched norms of Israeli non-obese children. Radial and tibial SOS was significantly reduced in obese compared to non-obese children (p < 0.05). Bone SOS in obese children with BMI > 95th percentile was not significantly different from obese children with BMI from 85-95th percentile. Tibial and radial SOS were correlated with pubertal stage (r = 0.52, p < 0.005; and r = 0.35, p < 0.01, respectively), and with chronological age (r = 0.47, p < 0.005; and r = 0.32, p < 0.025, respectively). No significant correlation was found between endurance time as an indicator of fitness and bone SOS. Bone strength measured by QUS is reduced in obese children, but is not affected by the severity of obesity.