Eighty patients who complained of retropatellar pain underwent evaluation by bone scintigraphy, intraosseous pressure determination, radiography, arthroscopy and physical diagnostic tests. The bone scans showed that 48% of the painful knees had an increased uptake compared with 9% for the normal joints. A highly significant correlation was evident between an increased uptake and established chondromalacia. For the diagnosis of a high pressure patella, radiography was only 7% sensitive (6/88), compared with 44% (39/88) for bone scintigraphy and 78% for the clinical "sustained flexion" test. The positive predictive value of a bone scan for detecting a high pressure patella was 0.72 (39/54). The best predictor was a positive sustained flexion test with a predictive value of 0.85 (69/81).