Tuberculosis causing an osteolytic lesion in the patella is very rare, having been reported on only four previous occasions. We present such a case and discuss the radiological features which help in differentiating tuberculosis from other similar lesions. A 25-year-old man in good health presented with a 3 month history of pain, swelling and restriction of movement of the right knee. There were no systemic symptoms or other joint involvement. Examination of the knee revealed an effusion with minimal terminal restriction of flexion. Radiographs of the knee joint demonstrated swelling (Fig. 1) and an osteolytic lesion in the patella (Fig. 2). Radionuclide bone scans showed an area of increased uptake in the right patella; no other lesions were seen on the scan. A chest radiograph revealed no abnormality.