Extensive fatigue tests were made on annealed Armco iron and plain carbon and alloy steels heat-treated to different strengths and microstructures. Statistics of fatigue-fracture curves and endurance limits were determined from the experimental data obtained and, for various other materials, from a survey of literature. The results were analyzed to show the relative effects of various metallurgical factors on the statistical nature of fatigue properties. Other phases of the problem studied include: dependence of statistical variation in fatigue life on stress level in the fracture range, statistics for location of crack initiation, size effect, understressing effect, and the form and method of plotting the S-N diagram.