The magnetocrystalline anisotropy energy of nickel has been measured in the temperature range 13.8 to 258K using a rotating sample magnetometer designed for use in solenoid. Measurements have been made in the (100) plane to deduce the anisotropy constants K₁ and K₃ and the uncertainty in these measurements is estimated to be less than 1% and about 10%, respectively. The values of K₁ are in good agreement with existing data, and are compared with recent band theory calculations, and with an empirical equation which takes account of conduction electron scattering. The values of K₃ are in broad agreement with existing data above about 80K but there is considerable disagreement at lower temperatures and a possible reason for this is advanced.