Purpose: To investigate the feasibility of sodium magnetic resonance (MR) imaging in the diagnosis of Achilles tendinopathy. Materials and Methods: Institutional review board approval and written informed consent were obtained. Twenty healthy volunteers and eight patients with Achilles tendinopathy were examined by using a 7-T whole-body MR imager with a 15-channel sodium knee coil. The sodium signal-to-noise ratio (SNR) from each region, as well as from the whole Achilles tendon, was compared between patients and healthy control subjects. The changes in SNR were assessed with a two-tailed unpaired t test in three regions of the Achilles tendon: the insertion area, the middle portion, and the muscle-tendon junction. P values less than .05 were considered to indicate a statistically significant difference. To validate a relationship between the sodium SNR and the glycosaminoglycan content in tendon, five cadaver ankles were examined with MR imaging and immunohistologically. The Pearson correlation coefficient between sodium SNR and glycosaminoglycan content was calculated. Results: Significant differences(P < .05) in the mean sodium SNR of healthy control subjects (mean SNR, 4.9 ± 2.1 [standard deviation]) and patients with chronic Achilles tendinopathy (mean SNR, 9.3 ± 2.3) were observed. Similar results were found at the insertion (mean SNR in control subjects, 6.7 ± 2.3; mean SNR in patients, 12.3 ± 4.5; P < .05) and the midportion (mean SNR in control subjects, 5.1 ± 1.9; mean SNR in patients, 9.4 ± 3.0; P < .05) of the Achilles tendon. At the muscle-tendon junction, the sodium SNR difference between control subjects and patients was small but still bellow the significance level (P = .0137). The increase in sodium SNR was observed in all regions independently of the location of morphologic findings. The Pearson correlation coefficient between sodium SNR and glycosaminoglycan content was 0.71. Conclusion: Sodium MR imaging may allow detection of the proteoglycan content increase in Achilles tendinopathy and thus identify the biochemical changes in the early stages of tendinopathy.