Radiological findings in 75 cases of aneurysmal bone cyst were analyzed. Sixty-five per cent were primary or simple and 35% were secondary, the aneurysmal bone cyst being combined with other osseous lesions. A primary aneurysmal bone cyst can be diagnosed with a high degree of certainty, but only 20% of secondary forms had the radiological appearance of aneurysmal bone cyst; in the other 80% the associated lesion dominated the radiological picture, particularly when it was malignant. In the secondary form a small biopsy specimen may show the features of aneurysmal bone cyst only; without radiological assistance a concomitant malignant lesion may be missed. Therefore, there must be close collaboration between the radiologist and and the pathologist.