A dense and transparent hydroxyapatite produced by hot isostatic pressing was investigated with respect to its stability and thermal expansion behaviour in the range 20–1150 °C. The evaluation was made by high-temperature X-ray diffraction in a reconstructed test chamber with a BN sample holder and Pt as an internal (in situ) reference. No degradation of the hydroxyapatite was detected in the range investigated. The lattice parameters, a and c, were determined as a function of temperature. The linear thermal expansion coefficient, determined by dilatometry, was 17.1 × 10^–6°C^–1, which agreed well with the value 17.3 × 10^–6°C^–1 obtained by X-ray diffraction.