Lifetime of hot-work tool steels is often limited by the development of crack nets as a consequence of thermal or mechanical fatigue loading. In this paper, the isothermal and thermal fatigue behaviour of tool steel AISI H11 (German grade X38CrMoV 5-1) in the temperature range between room temperature and 650°C is investigated. In addition to stress controlled isothermal tension-compression fatigue tests, out-of-phase thermal fatigue tests were carried out using a servohydraulic testing system, keeping the macroscopic total strain of the gauge length constant, while triangular time-temperature cycles were applied. The temperature range was chosen in such a way that numbers of cycles to fracture between 1,000 and 10,000 were achieved. The development of stress amplitudes, plastic strain amplitudes as well as of mean stresses was analysed during the tests. Results of isothermal and thermal fatigue tests are compared and discussed.