A rare case of parosteal aneurysmal bone cyst arising from the humerus in a 7-year-old boy is reported. Plain radiography and CT scanning revealed a marked periosteal reaction and a soft tissue mass surrounded by bone. Erosion of the cortex did not extend to involve the medulla. Three-dimensional CT scanning (3-DCT) revealed a birdcage-like ossified structure that was attached to the cortex of the humerus. 3-DCT was also used to visualize the extent of the tumor and the architecture of the bone, providing important information for planning of surgery. Inside the tumor, there were a number of fluid-fluid levels on T1-weighted MRI and high intensity areas on T2-weighted images. Pathologic examination revealed a central cavity containing a large volume of blood in the tumor and reactive osteogenesis mainly at the marginal regions, and these findings reflected the birdcage-like ossified structure observed by 3-DCT. Pathological examination was required to rule out osteosarcoma in particular, but a diagnosis of parosteal aneurysmal bone cyst was made because of the absence of findings suggestive of malignancy. The tumor was excised en bloc with a clear margin. At 3 years after surgery, there is no evidence of recurrent tumor.