A method, using in-core detector readings, is developed for estimating three-dimensional power distributions within boiling water reactors. The power distributions are obtained by a kind of synthesis technique which consists of two types of calculations, i.e., horizontal, two-dimensional whole core, and three-dimensional local core. The detector readings are used as internal boundary conditions. Power distributions obtained by this method compare satisfactorily with the exact solutions from whole-core calculations in test problems. The memory required to implement the technique is also acceptable to a process computer. Hence, the proposed method is suitable for on-line computer application.