Specific isothermal cyclic deformation tests were carried out on a tempered martensitic steel 55NiCrMoV7 at four hardness levels in the temperature range 20-600 °C. The cyclic stress response generally shows an initial exponential softening for the first few cycles, followed by a gradual softening without saturation. The influences of initial hardness obtained after tempering, temperature, strain rate and ageing on cyclic plasticity are discussed by means of hardness measurements and equivalent ageing experiments. Compared to hardness measured after ageing without fatigue loading, hardness dramatically decreases when the specimen is simultaneously subjected to ageing and fatigue at elevated temperature. Cyclic softening intensity increases with testing temperature from 300 to 600 °C, but the maximal softening intensity occurs at room temperature. This is inconsistent with hardness measurements performed on the specimen after a fatigue test conducted at room temperature. The final discussion focuses on the mechanisms involved in the softening of martensitic steels to explain the last result.