RATIONALE AND OBJECTIVES: The purpose of this study was to better understand the concept of mammography difficulty and how it affects radiology resident performance. MATERIALS AND METHODS: Seven radiology residents and three expert breast imagers reviewed 100 mammograms, consisting of bilateral medial lateral oblique and craniocaudal views, using a research workstation. The cases consisted of normal, benign, and malignant findings. Participants identified abnormalities and scored the difficulty and malignant potential for each case. Resident performance (sensitivity, specificity, and area under the receiver operating characteristic curve [AUC]) was calculated for self- and expert-assessed high and low difficulties. RESULTS: For cases classified by self-assessed difficulty, the resident AUCs were 0.667 for high difficulty and 0.771 for low difficulty cases (P = .010). Resident sensitivities were 0.707 for high and 0.614 for low difficulty cases (P = .113). Resident specificities were 0.583 for high and 0.905 for low difficulty cases (P < .001). For cases classified by expert-assessed difficulty, the resident AUCs were 0.583 for high and 0.783 for low difficulty cases (P = .001). Resident sensitivities were 0.558 for high and 0.796 for low difficulty cases (P < .001). Resident specificities were 0.714 for high and 0.740 for low difficulty cases (P = .807). CONCLUSIONS: Increased self- and expert-assessed difficulty is associated with a decrease in resident performance in mammography. However, while this lower performance is due to a decrease in specificity for self-assessed difficulty, it is due to a decrease in sensitivity for expert-assessed difficulty. These trends suggest that educators should provide a mix of self- and expert-assessed difficult cases in educational materials to maximize the effect of training on resident performance and confidence.