We report x-ray diffraction measurements on Ca(OH)2-portlandite as it is compressed to 37.6 gigapascals (GPa) in the diamond cell at room temperature. Between 10.7 and 15.4 GPa crystalline Ca(OH)2 transform to a glass, and on decompression the glass recrystallizes between 3.6 and 5.1 GPa. Below pressures of 10.7 GPa we measure the elastic compression of crystalline Ca(OH)2. A finite strain analysis of these data shows that the isothermal bulk modulus and its pressure derivative are K0=37.8(¿1.8) GPa and K'0=5.2(¿0.7) at zero pressure. From the change in the unit cell dimensions, we find that the linear incompressibilities of Ca(OH)2 differ by a factor of three. These data show that hydrogen bonding can produce large anisotropies in the elastic properties of hydroxides. We infer from the amorphization that dense, high-pressure phases of Ca(OH)2 exist at pressures of 11 GPa and above. ¿ American Geophysical Union 1990