A 7-year-old boy presented with an asymptomatic cystic lesion on the lateral aspect of the left ankle of 4 months’ duration. There was no history of trauma. The mass fluctuated in size; it was smaller when the child was recumbent and larger when the child was upright. Musculoskeletal examination of the spine, upper extremities, and lower extremities was entirely unremarkable, except for a tensely cystic mass over the lateral aspect of the left ankle just inferior to the lateral malleolus. The mass measured 3 × 1.5 cm. It nicely transilluminated when an otoscope was placed over its top. The mass did not change in size when the peroneal tendons were stressed or when the foot moved through a full range of motion. A sonogram of the left ankle confirmed the presence of a lobulated elongated cyst of 3 × 1.5 cm. Alexander K. C. Leung, MD, and Justine H. S. Fong, MD, of Calgary, Alberta, diagnosed a peroneal ganglion. This benign, fluid-filled cyst results from either herniation of the synovium through a defect in a joint or myxomatous degeneration of the connective tissue close to a joint or tendon. The lesion is typically connected to the underlying joint capsule or tendon sheath by a stalk that may be tortuous. Transillumination with an otoscope is an easy way to establish the diagnosis. Ultrasonography can help to better define the mass. The lesion is usually asymptomatic but may become tender as it expands or when it is impacted. Sites of predilection include the dorsum of the wrist, dorsum of the foot, and the ankle. In children, ganglia are usually benign and tend to disappear over time. Operative excision is recommended if the lesion is painful, unsightly, interferes with normal tendon function, or rapidly increases in size. Because none of the indications existed in this patient, therapy was conservative.