Determining the amount of scatter inside and outside a diagnostic X-ray room is important for evaluating the dose to staff and the public. The amount of scatter is affected by many physical factors including beam quality and field size. However, there is little published data on patient scatter and there are large differences between the available data sets. Hence, a Monte Carlo code was developed to allow a systematic study of the factors affecting patient scatter. A voxel phantom was used to provide a realistic model of the patient. The variation of scatter with different phantom types was investigated to show the effect of patient inhomogeneities and obliquities. The effect of altering tube voltage, filtration, voltage ripple, field size and position on patient scatter was studied. A larger than expected variation in the patient scatter was observed with increasing field area due to the proximity of the field borders with the patient obliquities. The effect of the tube voltage ripple on the patient scatter was also calculated. This showed that there would be little effect on the scatter levels within X-ray rooms if ageing X-ray generators, which produce substantial voltage ripple, were replaced by X-ray tubes with modern medium frequency generators. Recommendations are made on the choice of published scatter data for X-ray room design.