Acoustic speed is an important parameter that can be used to characterize bone tissue for evaluation of osteoporosis. Traditional approaches for measuring acoustic speed require knowledge of either the specimen thickness, which is sometimes dlfllcult to obtain from biological tissues, or the reference fluid velocity that, moreover, is an unknown parameter for internal tissue in in vivo measurement. In this paper, a new method is proposed to measure acoustic speed from dual reflected ultrasound signals. This technique utilizes two transducers placed on the same side of the test object; one for transmitter and receiver, and the other one for receiver only. The acoustic speed of the test object is based on the information of time-of-flight from the signals received by both transducers and the separation distance between the transducer pair. The technique developed here not only eliminates the requirement of knowledge of specimen thickness, but also shows a feasibility for clinical applications. The results of different porcine and bovine bone samples measured in vitro by this technique are in good agreement with those measured by other published methods. In vivo measurement results of 10 healthy young volunteers' tibias are also reported. 0 1997 World Federation for Ultrasound in Medicine & Biology.