Magnetic resonance imaging (MRI) is a technology frequently used to evaluate low back pain, despite evidence that challenges the usefulness of routine MRI and the surgical interventions it may trigger. We analyze the relationship between MRI supply and care for fee-for-service Medicare patients with low back pain. We find that increases in MRI supply are related to higher use of both low back MRI and surgery. This is worrisome, and careful attention should be paid to assessing the outcomes for patients.