Fifty patients presenting themselves with a locked knee were investigated prospectively by examination under anaesthesia and arthroscopy. Definite abnormality was found in 92 per cent. Torn menisci were present in 68 per cent and isolated ruptures of the cruciate ligament in 10 per cent. Loose bodies were found in 3 patients, degenerative changes alone in 2 patients and a pathological medial synovial shelf in 1 patient. No abnormality was demonstrated in 8 per cent. Sixteen per cent of the knees remained locked following the induction of anaesthesia. This study demonstrates that a true mechanical block is not necessarily present and that the position of an unstable meniscal fragment is not consistently related to the fixed position of the knee under anaesthetic. There were no clinical features that allowed the normal knees to be distinguished preoperatively from those with internal derangement. It is proposed that these patients should be managed by prompt examination under anaesthesia, arthroscopy and definitive surgery.