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Monte Carlo procedures using the SIERRA code (validated in a companion article) were used to investigate the scatter properties in mammography. The scatter to primary ratio (SPR) was used for quantifying scatter levels as a function of beam spectrum, position in the field, air gap, breast thickness, tissue composition, and the area of the field of view (FOV). The geometry of slot scan mammography was also simulated, and SPR values were calculated as a function of slot width. The influence of large air gaps (to 30 cm) was also studied in the context of magnification mammography. X-ray energy and tissue composition from 100% adipose to 100% glandular demonstrated little effect on the SPR. Air gaps over a range from 0 to 30 mm showed only slight effects. The SPR increased with increased breast thickness and with larger fields of view. Measurements from 82 mammograms provided estimates of the range of compressed breast thickness (median: 5.2 cm, 95% range: 2.4 cm to 7.9 cm) and projected breast area onto the film (left craniocaudal view, median: 146 cm2, 95% range: 58 cm2 to 298 cm2). SPR values for semicircular breast shapes, Mo/Mo spectra, and a 15 mm air gap were parametrized as a function of breast thickness and (semicircular) breast diameter. With the coefficients a = - 2.35452817439093, b = 22.3960980055927, and c = 8.85064260299289, the equation SPR= [a + b x (diameter in cm)--(-1.5) + c x (thickness in cm) --(-0.5)]-- -1 produces SPR data from 2 to 8 cm and from 3 to 30 cm breast diameters with an average error of about 1%.