The fundamental structure parameter determining the properties of titanium alloys is the grain size. The grain size can be controlled by means of the lattice strain and the internal work-hardening which accompany the phase transitions occurring during repeated heating <----> cooling cycles. During cyclic heat treatments, there are conditions facilitating the cumulation of internal work-hardening. The structural research carried out has shown that in the pre-treatment cycles the structural defects refer mainly to the alpha phase and manifest itself by an increase in the microtwin density. Larger numbers of cycles increase the dislocation density mainly in the alpha phase and then in the beta phase. (C) 2003 Elsevier Science B.V. All rights reserved.