Mammography x-ray tube focal spot sizes are routinely measured during acceptance testing and annual performance audits. The National Electrical Manufactures Association (NEMA) recommends the slit camera for this purpose. Investigated were the effect of slit rotational misalignment, tilt misalignment, image film density, film and screen-film image receptors, microscope magnification and reticule accuracy, and observer variation on slit camera focal spot measurements. Our results indicate that small rotational misalignment (< 5 degrees) and tilt misalignment (< 3 degrees) introduce insignificant error. Measured focal spot size increased slightly with image optical density, indicating that for consistent results the image optical density variations should be minimized. Also desirable for accurate field measurements is a high power microscope (25-50x) and a reticule with divisions of < or = 0.02 mm. Screen-film imaging consistently resulted in a slightly smaller measured focal spot size than direct film. The greatest source of error was due to observer variation. Of interest is that reader variability showed a consistent pattern and variation between two measurements by the same observer was much smaller than between observer variation, suggesting that standardized criteria should be established and a method of reader training developed. The length of the focal spot is defined at a reference axis angle specified by the mammography unit manufacturer. Presented is a tabulation of the focal spot geometry and reference axis angles for the majority of mammography units currently and recently marketed in North America.