Purpose: To investigate the mammographic presentation of the pectoral muscle on the mediolateral oblique view of the breast and its relationship to current image evaluation criteria. Methods: A retrospective review of 100 paired mediolateral oblique mammograms performed on women undergoing routine mammographic screening with BreastScreen NSW South West, Australia was undertaken. Measurements of length, width, contour type and inferior angle of the pectoral muscle and of the relationship of its inferior aspect to the posterior nipple line and the nipple level were recorded. Results: A wide range of parameter values representing acceptable imaging were found to exist for each of the key imaging criteria. In 75.5% of images reviewed the inferior aspect of the pectoral muscle did not reach the level of the nipple, failing the quality criterion of the image evaluation systems used by the UK National Health Service Breast Screening Programme and the Australian BreastScreen Program. Conclusion: It is recommended that aspects of current image evaluation criteria relating to the presentation of the pectoral muscle should be modified to include quantified ranges to reduce inherent subjectivity and variability in interpretation. The criterion for the inferior aspect of the pectoral muscle to reach the level of the nipple should be reviewed by the national accreditation bodies.