Cerebrotendinous xanthomatosis is a rare autosomal recessive lipid-storage disease caused by a mutation in the sterol 27-hydroxylase (CYP27) gene1,2. It is important that orthopaedic surgeons be aware of this condition because the initial presentation may be symmetric, painful enlargement and deformity of the Achilles tendons. Early diagnosis is the key to treatment because medical therapy is effective in halting progression of, although not reversing, the devastating neurological lesions of this condition. The subject of this case report was aware that data concerning the case would be submitted for publication. A thirty-one-year-old man presented to the clinic of the senior author (J.W.B.) with a six-year history of bilateral, slowly progressive, painful swelling of the Achilles tendon. He stated that it interfered with his ability to walk, which was already affected by a neurological condition. The pain was exacerbated by walking, was relieved somewhat by rest, and at the time of presentation restricted the patient's walking distance to a maximum of two city blocks. A neurologist had previously diagnosed multiple sclerosis on the basis of the clinical findings of tremor, sensory neuropathy in the extremities, an ataxic gait, and plaque-like changes in the cerebral cortex on magnetic resonance imaging. The patient had a family history of type-2 diabetes mellitus but no other inherited disorders. Fig. 1 Clinical photograph demonstrating bilateral symmetrical fusiform swelling of the Achilles tendon. Figs. 2-A and 2-B Sagittal T1-weighted magnetic resonance imaging scan demonstrating fusiform swelling of the right Achilles tendon just proximal to its insertion. All other tendons around the foot and ankle appear normal. Axial T1-weighted image of the right Achilles tendon 2.5 cm proximal to the tibial plafond. A heterogeneous signal can be observed in the tendon. Remaining fibers of normal tendon (black) are interspersed between areas of intermediate signal (gray), representing inflammatory tissue, and high signal …