An investigation was undertaken to evaluate the effects of mean stress on the high cycle fatigue (HCF) limit stress under uniaxial loading. Tests were conducted at constant values of stress ratio, R (ratio of minimum to maximum stress), at frequencies from 20 to 70 Hz up to 107 cycles using a step-loading technique developed by the authors. Data were presented in the form of a Haigh (Modified Goodman) diagram as alternating stress against mean stress. Tests in the regime R < - 1 were conducted to determine the effect of negative mean stresses on the material behavior. The lowest mean stress corresponded to R = - 4, below which the compressive yield stress of the material would be exceeded. While numerous models could provide approximate fits to the data in the constant life Haigh diagram for positive mean stresses, none of them captured the trends of the data over the entire mean stress range including R < - 1. The Jasper equation, based on a constant range of stored energy density, was found to represent the positive mean stress data quite well. The equation was modified to account for stored energy density at negative mean stresses. The best fit to the data implies that compressive strain energy density contributes less than 30 percent to the fatigue process as compared to energy under tensile stresses. Further, initiation to a fixed crack length beyond which crack propagation occurs does not explain the shape of the Haigh diagram. It is concluded through simple analysis that there is no clear link between HCF crack initiation, which represents a majority of life in some applications, and crack growth threshold, and the two might represent entirely different mechanisms.