We tested the accuracy and feasibility of using cine phase contrast magnetic resonance imaging (cine-PC MRI) to non-invasively measure three-dimensional, in vivo, skeletal velocity. Bone displacement was estimated by integrating the velocity measurements. Cine-PC MRI was originally developed to directly and non-invasively measure in vivo blood and heart velocity. Since no standard of reference exists for in vivo measurement of trabecular bone motion, a motion phantom (consisting of a series of paired gears that moved a sample box containing a human femoral bone sample) was built to assess the accuracy of tracking trabecular bone with cine-PC MRI. The in-plane, average absolute displacement errors were 0.55±0.38 and 0.36±0.27 mm in the x- and y-direction, respectively. Thus, estimates of bone position based on the integration of bone velocity measurements are affected little by the magnetic properties of bone [Majumdar and Genant (1995) Osteoporos International 5, 79–92]. The velocity profiles of the patella, femur and tibia were measured in five healthy subjects during leg extensions. Extension was resisted by a 34 N weight. Subjects maintained a consistent motion rate (35±0.5 cycles min⁻¹) and motion artifacts were minimal. Our results indicate that patellar flexion lags knee flexion and the patella tilts laterally and then medially as the knee extends. We conclude cine-PC MRI is a promising technique for the non-invasive measurement of in vivo skeletal dynamics and, based on our previous work, muscular dynamics as well.