The diagnostic performance of ultrasonography (US) in the detection of partial ruptures in the proximal part of the patellar ligament (jumper's knee) was studied. A total of 81 athletes with chronic localized pain suggestive of jumper's knee underwent US examination, and 25 of these received surgical treatment. Of 25 proven partial tendon ruptures at surgery, US correctly indicated the diagnosis in all cases. A cone-shaped, poorly echogenic area exceeding 0.5 cm in length in the center of the patellar tendon in combination with its localized thickening proved to be a reliable indicator of jumper's knee. One case was a true-negative. No false-negative or false-positive case was observed. Soft-tissue radiography in 14 cases showed a localized swelling but did not detect intratendinous soft-tissue abnormalities. US is the method of choice for the evaluation of jumper's knee, as it is cheap, non-invasive, repeatable, and accurate.