Background: Quadriceps atrophy and in particular atrophy of the vastus medialis obliquus (VMO) muscle have been frequently related with patellofemoral pain syndrome (PFPS), despite very little objective evidence. Hypothesis: Patients with PFPS exhibit atrophy of the VMO in comparison with healthy controls. Study Design: Case-control study; Level of evidence, 3. Methods: Forty-six patients with PFPS and 30 healthy control persons with similar age, gender, body mass index, and activity index distributions underwent magnetic resonance imaging (MRI) of the quadriceps. The muscle size was determined by calculating the cross-sectional area of the total quadriceps and its components. Results: The cross-sectional area (CSA) of the VMO was significantly smaller in the PFPS group than in the control group (16.67 +/- 4.97 cm(2) vs 18.36 +/- 5.25 cm(2)) (P = .040). A tendency was noted for a smaller total quadriceps CSA for the PFPS patients at midthigh level (66.99 +/- 15.06 cm(2) vs 70.83 +/- 15.30 cm(2)) (P = .074). Conclusion: This is the first study to examine VMO size in PFPS patients by MRI. Patients with patellofemoral problems exhibited atrophy of the VMO. Although it is not clear whether this atrophy is a result or a cause of PFPS, the results of this study do show that atrophy of the VMO is a contributing factor in PFPS. Longitudinal, prospective studies are needed to establish the cause-effect relation of VMO atrophy and PFPS.