A NiTi alloy consisting of 50.7 at. % Ni was studied under cyclic loading. This alloy can show pseudo-elastic behaviour at room temperature (M-s < RT < M-d). It was produced in different microstructural conditions by thermal or thermo-mechanical treatments. Special attention was paid to the heterogeneity of strain caused by the stress induced phase transformation during cycling of the different microstructures, The behaviour after repeated mechanical cycles was analysed. The change of mechanical hysteresis in different microstructures was investigated. The effects of primary defects (as introduced by pretreatment) were related to the microscopic and macroscopic fatigue behaviour. The thermo-mechanically treated states [1] show the greatest resistance against fatigue damage.