In order to clarify the mechanism of ball throwers’ humeral fractures, two types of mechanical tests were done. Torsional or bending load was applied to produce experimental fractures of in eight canine whole humeri. Canine humeri were easily broken by torsion, but could tolerate tensile stress by bending. The ratio of torsional strength to bending strength was calculated to be 0.156. Our ten clinical cases and 118 cases in Japanese literature were reviewed on radiograms and clinical situations of injury: position, type of throw, kind of pitches, type of ball. All cases were amateurs and no professional players. No clinical factors on these situations were specified. All fractures were brittle and spiral in shape and were inter-preted to be produced not by bending, but by torsion. When we combine the mechanical analysis in our experiment with review of the data in 172 clinical fracture cases, the following conclusion is obtained: throwers’ fractures can be produced by torsion of relatively small torque, while they aren't induced by bending even by a maximal muscular strength in a skilled pitcher. The key point of prevention is education about correct pitching forms to minimize torsion to the humerus.