Most crystallization experiments of macromolecules are carried out at a constant temperature. Room temperature (22 degreesC) and 4 degreesC are the most widely used settings in crystallization. In practice, crystal growth at relatively high temperatures has often been avoided for macromolecular crystallization. Human estrogenic 17 beta -hydroxysteroid dehydrogenase has been crystallized in complex with dehydroepiandrosterone or dihydrotestosterone. The crystallization experiments were carried out at 27 degreesC. The 17 beta -HSD1 crystals were greatly improved at the elevated temperature. The effects of higher temperatures on crystal growth were studied. High temperatures stimulated the nucleation of 17 beta -HSD1, increased the rate of crystal growth, and higher occupancy of substrates was obtained in the crystal structure. This method also reduced the formation of twin crystals. Since temperature is the easiest factor to control in the laboratory, crystallization at elevated temperatures provides an efficient method to improve protein crystal growth. The mechanism of the effect of temperature and relative techniques are discussed.