Intraosseous pneumatocysts are benign air-containing lesions that are most often found in the spine and pelvis and are nearly always treated nonoperatively. Although rarely clinically symptomatic, studies have shown pneumatocysts to be present in up to 10% of computed tomography (CT) scans of the pelvis and spine. Radiographic characteristics of these lesions include a localized collection of gas with a thin sclerotic rim, no bony destruction, no soft tissue masses, and no medullary abnormalities. Computed tomography is the diagnostic study of choice, with Hounsfield units ranging from -580 to -950, showing a gas-containing lesion. Few studies have described the management of symptomatic pneumatocysts, and all reported cases concern underwater divers, presumably because of greater pressure cycling and barotrauma encountered while underwater diving. The goal of this report is to describe the intraoperative CT-guided navigation and percutaneous injection of calcium sulfate-calcium phosphate composite bone graft substitute material for the treatment of a symptomatic pneumatocyst in the ilium of a Navy dive instructor. The patient reported a 1-year history of increasing buttock pain with increased depth of diving, consistently reproduced by diving past a depth of 20 to 30 feet. To the authors' knowledge, this is the first description in the English literature of the operative treatment of an intraosseous pneumatocyst of the ilium. The use of intraoperative CT guidance permitted accurate percutaneous localization, decompression, and filling of the lesion with synthetic bone graft substitute, with complete early relief of symptoms. At 6-month follow up, the patient had reached diving depths of 170 feet without pain.