BACKGROUND: Posterior tibial tendon dysfunction (PTTD) is a common malady leading to acquired flatfoot, which frequently causes foot and ankle pain and swelling. We hypothesized that patients with PTTD and pitting edema along the course of the posterior tibial tendon (PTT) would have evidence of fluid within the tendon sheath on MRI. We introduce and validate a novel, yet simple physical examination finding, the posterior tibial edema (PTE) sign, to help distinguish PTTD from other causes of foot and ankle pain and swelling. PATIENTS AND METHODS: To determine an association between the PTE sign and changes to the PTT evident on MRI, we reviewed the medical records of patients with PTTD who underwent magnetic resonance imaging (MRI). RESULTS: Of 186 patients identified with PTTD, 49 underwent MRI, and 42 (86%) demonstrated imaging characteristics consistent with tendonitis, tendinosis, or tenosynovitis. Presence or absence of increased posterior malleolar pitting edema agreed with presence or absence of these MRI findings in 43 patients (88%), making the clinical examination finding significantly predictive of MRI results (p= 0.01). The sensitivity of the test for PTE was 86%, with specificity of 100%. CONCLUSION: The PTE sign refers to pitting edema along the course of the PTT, not associated with acute trauma and not accompanied by other areas of edema. Physical examination signs that are highly sensitive and specific to the condition may help lower the cost of diagnosis by eliminating the need for expensive tests that are not always necessary.