A method is presented for calculating the temperature increase that results from heat generation by radioactive wastes placed in a subsurface cavity. Cavities of spherical, cylindrical, and other shapes are considered. The method takes into account the disintegration energy released by the various isotopes present in a waste and is applicable to wastes of all types, irrespective of their origin, age, or radioactive composition. This has been achieved by grouping the isotopes according to their decay characteristics. The procedure described is particularly useful when dealing with high-level wastes from power reactors. After a detailed examination of all the isotopes identified in significant proportions in a 90-day-old waste, it is demonstrated that when such a waste becomes 1 year old there are only 5 isotope groups that are mainly responsible for the heat generation. The number of groups reduces to 4 when the waste becomes 2 years old. In the theoretical treatment of the problem dimensionless parameters are used. These parameters make it possible to present the results in a few diagrams, which can be easily used for a rapid determination of the temperature rise for a subsurface cavity containing radioactive waste. Since these diagrams do not depend on specific values of cavity dimensions, thermal constants of subsurface materials, or decay characteristics of the radioactive isotopes, they are applicable to a wide variety of situations that may be encountered in practice. By comparing the temperature rise from a subsurface heat source of constant strength with another that decays exponentially with time, the necessary conditions are determined for considering that the radioactive waste is a constant source of heat without introducing serious errors. As an application of the theory, the temperature field from a spherical cavity in rock salt containing a 2-year-old reactor waste has been investigated in detail. The space and time variation of the temperature, the maximum temperatures at various points, the rate of variation, the extent of penetration of the field, and the influence of this penetration on major geologic structures such as a salt dome are discussed.