Purpose: To determine the accuracy of unenhanced magnetic resonance (MR) imaging in the detection of acute appendicitis in patients younger than 50 years who present to the emergency department with right lower quadrant (RLQ) pain. Materials and Methods: The institutional review board approved this retrospective study of 403 patients from August 1, 2012, to July 30, 2014, and waived the informed consent requirement. A cross-department strategy was instituted to use MR imaging as the primary diagnostic modality in patients aged 349 years who presented to the emergency department with RLQ pain. All MR examinations were performed with a 1.5-or 3.0-T system. Images were acquired without breath holding by using multiplanar half-Fourier singleshot T2-weighted imaging without and with spectral adiabatic inversion recovery fat suppression without oral or intravenous contrast material. MR imaging room time was measured for each patient. Prospective image interpretations from clinical records were reviewed to document acute appendicitis or other causes of abdominal pain. Final clinical outcomes were determined by using (a) surgical results (n = 77), (b) telephone follow-up combined with review of the patient's medical records (n = 291), or (c) consensus expert panel assessment if no follow-up data were available (n = 35). Logistic regression analysis was performed to evaluate the sensitivity and specificity of MR imaging in the detection of acute appendicitis, and corresponding 95% confidence intervals were determined. Results: Of the 403 patients, 67 had MR imaging findings that were positive for acute appendicitis, and 336 had negative findings. MR imaging had a sensitivity of 97.0% (65 of 67) and a specificity of 99.4% (334 of 336). The mean total room time was 14 minutes (range, 862 minutes). An alternate diagnosis was offered in 173 (51.5%) of 336 patients. Conclusion: MR imaging is a highly sensitive and specific test in the evaluation of patients younger than 50 years with acute RLQ pain that uses a rapid imaging protocol performed without intravenous or oral contrast material.