A 15-year-old boy with Crohn's ileocolitis developed marked gallbladder enlargement. Ultrasonographic findings were consistent with acalculous cholecystitis (AAC) or hydrops. At laparotomy a gangrenous gallbladder was found. The diagnostic modalities currently used to distinguish between hydrops, a benign condition generally treated expectantly, and AAC, a potentially life-threatening condition requiring surgical treatment are reviewed. Failure to distinguish between these two conditions with acalculous gallbladder enlargement and similar clinical and radiologic features may have serious consequences.