Chronic patellar tendinosis or "jumper's knee" is a frequent source of persistent and often recurrent anterior knee pain in active, young people. Seventeen pathologic specimens retrieved during surgery for jumper's knee were evaluated by means of immunohistochemical (S-100) analysis. The patellar tendon-bone junction and the Hoffa fat pad adjacent to the inferior pole of the patella are structures that have a nerve supply that can be a potential source of nociceptive output resulting in the perception of pain at any given moment. In this study, pathologic neural changes also were observed. In eight cases, free nerve endings showed a histologic pattern of "nerve sprouting" in the patellar tendon-bone junction. Vascular innervation was seen in seven cases. S-100 positive fibers were observed within the muscular layer of medium and small arteries. These findings show an increase in vascular innervation. Lastly, neuromatous changes were observed in four cases, demonstrating a clear relationship with pain. These observations provide a neuroanatomic basis for pain in active, young patients with jumper's knee.