A rotation-based Monte Carlo (MC) simulation method (RMC) has been developed, designed for rapid calculation of downscatter through non-uniform media in SPECT. A possible application is downscatter correction in dual isotope SPECT. With RMC, only a fraction of all projections of a SPECT study have to be MC simulated in a standard manner. The other projections can be estimated rapidly using the results of these standard MC calculations. For efficiency, approximations have to be made in RMC with regard to the final scatter angle of the detected photons. Further speed-up is obtained by combining RMC with convolution-based forced detection (CFD) instead of forced detection (FD), which is a more common variance reduction technique for MC. The RMC method was compared with standard MC for 99mTc downscatter in a 201Tl window (72 keV+/-10%) using a digital thorax phantom. The resulting scatter projections are in good agreement (maximum bias a few per cent of the largest value in the projection), but RMC with CFD is about three orders in magnitude faster than standard MC with FD and up to 25 times faster than standard MC with CFD. Using RMC combined with CFD, the generation of 64 almost noise-free downscatter projections (64 x 64) takes only a couple of minutes on a 500 MHz Pentium processor. Therefore, rotation-based Monte Carlo could serve as a practical tool for downscatter correction schemes in dual isotope SPECT.