Within the accuracy of the scalar wave equation, a novel beam propagation method (BPM) is proposed for the calculation of integrated optical waveguide structures with large refractive index steps DELTA-n. Propagation increments DELTA-z in excess of a wavelength are possible for DELTA-n almost-equal-to 2.5. In this way, high stability and accuracy can be maintained over a large total propagation length. The calculation time is kept low by using a finite-difference scheme.