The objective of this study was to develop a non-destructive method for monitoring fracture healing with acoustic emission (AE). Experimentally produced fractures of the rat femur were tested in tension and in torsion at 4, 6, 8 and 12 weeks after fracture. AE signals were monitored during these mechanical tests. The values for load and torque at the initiation of the AE signal were defined as new mechanical parameters. The apparent density and ash density of the fracture site were also measured at each time period. Tensile strength, tensile stiffness, maximum torque and torsional stiffness of the fracture site increased with time. The AE signal was detected before complete specimen failure. Load and torque for initiation of AE increased proportionally with increasing mechanical properties. The mineral density, however, reached a plateau at 8 weeks, when callus mechanical strength was approximately 50% of control. Load for initiation of AE was strongly correlated with the strength (r = 0.98), stiffness (r = 0.88), and failure strain (r = –0.63) of the callus. Torque for initiation of AE was highly correlated with the maximum torque (r = 0.95) and torsional stiffness (r = 0.93) of the callus. The findings of the present study indicated that some mechanical properties of healing fractures could be estimated by monitoring AE signals. © 2001 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.