PURPOSE: To prospectively evaluate with magnetic resonance (MR) imaging the acute changes in the heel associated with extracorporeal shock wave therapy (ESWT). MATERIALS AND METHODS: Institutional clinical study review board approved the study, and informed consent was obtained. MR imaging was performed within 24 hours before and after ESWT on 18 feet of 12 patients (eight women and four men; age range, 33-63 years; average, 49.9 years) with chronic plantar fasciitis. ESWT was applied to the most painful point on the plantar surface of the heel, with a total of 1500 shocks at 18 kV. The MR imaging protocol consisted of sagittal and coronal T1- and T2-weighted images with and without fat saturation. The images were reviewed to assess the post-ESWT changes in soft-tissue and bone marrow edema, the thickness of the proximal plantar fascia, and the presence of a heel spur. Paired t test was used for the statistical analysis. RESULTS: Soft-tissue edema, which was present in 16 (89%) of 18 heels before ESWT, had increased in severity in 12 (75%) heels after ESWT. Calcaneus bone marrow edema at the insertion site was observed in eight heels before ESWT. After ESWT, the extant of bone marrow edema had increased in one heel and had newly developed in another heel. The heel spur seen in nine (50%) feet was not affected by ESWT. In 17 (94%) heels, the proximal plantar fascia was abnormally thick, with thickness not significantly changed with use of ESWT (P > .05). CONCLUSION: Increase in soft-tissue edema is the most common acute response associated with ESWT.