An efficient algorithm for pattern matching has been developed based on least-squares analysis of fitting a discrete set of master patterns against measured images. This algorithm has been applied to determine three-dimensional molecule orientations in defocused single-molecule images. The developed algorithm exploits the excellent agreement between electrodynamic calculations of single-molecule emission and experimentally measured images. The procedure is found to be reliable and simple and can be applied to any kind of pattern recognition where the patterns to be recognized are precisely known a priori. The procedure works well even for noisy and low-intensity signals as usually encountered in single-molecule experiments.