This study investigated the prevalence of each of the four features of patellar tendinosis in asymptomatic athletic subjects undergoing patellar tendon anterior cruciate ligament (ACL) reconstruction. Fifty subjects (39 males and 11 females) undergoing ACL reconstruction using a patellar tendon graft were screened for previous tendon symptoms, training and playing history and had their patellar tendons examined with ultrasound prior to surgery. During surgery, a small piece of proximal posterocentral tendon was harvested, fixed and examined under light microscopy. Histopathological changes were graded for severity. Results demonstrate that 18 tendons were abnormal on light microscopy and 32 were normal. There were no differences between subjects with and without pathology in respect of training, recovery after surgery and basic anthropometric measures. Three tendons were abnormal on ultrasound but only one had proximal and central changes. Tendons showed a consistent series of changes. Tenocyte changes were found in all but one of the abnormal tendons. In all but one of the tendons with increased ground substance there were tenocyte changes, and collagen separation was always associated with both tenocyte changes and increased ground substance. No tendons demonstrated neovascularization. It appears that cellular changes must be present if there is an increase in ground substance, or collagen and vascular changes. Further research is required to confirm these findings.