Clinical breast examinations (CBE) play a role in the detection of breast cancers. However, most physicians receive inadequate training in tactile search of breast tissue to detect small (< 2 cm), hard tumors. The dynamic, variable-lump, silicone breast simulator was designed to improve physicians' CBE performance and increase tumor detection. Water inflates balloons embedded in formed silicone to simulate the presence of tumors and allow independent adjustment of tumor hardness. The advantage, compared to static models with five, non-movable tumors, is that training scenarios can be reconfigured and repeated until each trainee learns the subtle tactile cues associated with tumors. In a study of 48 medical students, training with the dynamic simulator increased the number of tumors found (F(42)=7.85, p=0.0077), reduced the number of false positives (F(42)=5.20, p=0.0277), and improved transfer of training. This advancement can allow CBE to become more reliable, consistent, and effective.