PURPOSE: To evaluate the usefulness of magnetic resonance imaging(MRI) and ultrasonography(US) for thediagnosis of jumper's knee in high school basketball players. MATERIALS AND METHODS: In order to detect clinicalcases of jumper's knee, 40 sites of patellar tendon in 20 knees of ten basketball players were assessed by anevaluation of personal history and physical examination. US and MRI were used for detecting compatible findings ofthis condition ; the diagnostic criteria were hypoechoic focus with focal thickening as seen on US, and increasedsignal intensity with focal thickening as seen on MRI. As an early finding of jumper's knee a new sonographiccriterion of focal hypoechoic focus with or without focal thickening, was also applied. RESULTS: At 19 of 40sites(48%), clinical jumper's knee was diagnosed. For the detection of this condition according to known cliteria,sensitivity and specificity were, respectively, 63% and 100%(US) and 32% and 90%(MRI). Using the new criterion,the sensitivity and specificity of US were 84% and 76%, respectively. CONCLUSION: On the basis of known US andMRI criteria for jumper's knee, the sensitivity and specificity of US were higher than those of MRI. We suggestthat hypoechoic focus without focal thickening oas seen on US, is an early finding of jumper's knee.