Background <p>Lateral epicondylitis (LE) is characterized by enthesopathy and tendon degeneration of the extensor carpi radialis brevis (ECRB) attachment. Although magnetic resonance imaging (MRI) can reveal pathological changes, quantitative evaluation methods remain limited. This study aimed to quantify ECRB tendon degeneration on computed tomography (CT) using Hounsfield unit (HU) values.</p> Methods <p>We analyzed 24 elbows (12 males, 12 females; mean age: 53.7 years) in the LE group and 25 control elbows (16 males and 9 females; mean age: 56.7 years). All the patients in the study group underwent preoperative CT, MRI, and subsequent ECRB tendon resection. The tendon was divided into proximal, middle, and distal regions. The mean HU values were measured on reconstructed coronal CT images and analyzed in relation to the control group measurements, MRI findings, and histological evaluations.</p> Results <p>The mean HU values were significantly lower in the LE group than in the controls in the proximal (49.7 vs. 65.4 HU, <i>P</i> &lt; .0001) and middle regions (58.9 vs. 67.1 HU, <i>P</i> = 0.024) but not in the distal region (66.1 vs. 70.8 HU, <i>P</i> = 0.192). The ECRB tendon showed the greatest histological degeneration proximally, demonstrating a strong negative correlation with HU values (Spearman ρ = -0.612, <i>P</i> &lt; .0001). Proximal region HU values showed a moderate negative correlation with MRI grades (Spearman ρ = -0.517, <i>P</i> = 0.020).</p> Conclusion <p>HU values provide a quantitative method for evaluating ECRB tendon degeneration in LE, correlating well with histological and MRI findings. This technique offers an objective measure of tendon pathology that may complement the current diagnostic approaches.</p> Evidence from the study <p>Level III.</p>