Tests of a Cr-Mo-V rotor steel have been carried out at 550°C using continuous cycling and cycling with tensile dwells. A special feature of these tests was that they were carried out in control by the axial total strain but that diametral strain was simultaneously measured. Comparison of the diametral and axial strain ranges showed that “Poisson's ratio” initially increased but subsequently declined steadily with accumulating cycles. While these results do not challenge the traditional view of Poisson's ratio which is defined for homogeneous material, they indicate that numerical values of the ratio must be used with caution. The results on diametral strain response are carefully analyzed and defended as a true material response; they indicate that the ratio of diametral to axial strain is sensitive to the accumulated fatigue damage of the specimen in a rather unpredictable fashion.