BACKGROUND: Magnetic resonance imaging (MRI) is commonly used to diagnose and assess the extent of partial distal biceps injuries. The aim of this study was to report on the accuracy of MRI and the effect of injury history and study timing on its performance. METHODS: A retrospective review of all patients who underwent surgical treatment of partial thickness distal biceps tears at a single center by multiple surgeons was performed. Inclusion criteria consisted of the performance of a preoperative MRI and documentation of the intraoperatively visualized extent of the tear, and 68 patients met the criteria for inclusion. A chart review was completed to evaluate the symptom duration, injury history, and tear extent. RESULTS: All patients had distal biceps tears of greater than 50% intraoperatively. However, MRI did not indicate any tearing in 20 (29%) patients, and its sensitivity for high-grade tear was 44%. Magnetic resonance imaging was significantly less likely to be read as high-grade tears in patients with insidious onset of their symptoms in comparison with patients who reported a traumatic onset (27% vs 55%, CONCLUSIONS: These results indicate that MRI can underreport partial distal biceps tear extent, and this more commonly occurs in patients with insidious onset of pain.