Stress fractures of the proximal ulna are known to occur in throwing athletes. Most cases extend to involve the olecranon, and cases limited to the trochlear groove are rare. In this report we present a 17-year-old elite baseball pitcher with a stress fracture of the trochlear groove of the proximal ulna. Diagnosis was made by demonstration of characteristic signal changes on MRI of the elbow. The fracture occurred at the cortical notch, also known as the pseudodefect of the trochlear groove. This case suggests that the cortical notch serves as an area of weakness predisposing pitchers to development of a stress fracture.