Lhermitte's sign is a common early symptom of subacute combined degeneration of the cord occurring in 11 out of 44 patients admitted to the National Hospitals for Nervous Diseases during the decade 1962-71 with this diagnosis. Two patients, in both of whom it was the presenting complaint, are described in detail. It is concluded that, in these cases, Lhermitte's sign is due to stretching of demyelinated fibres in the posterior columns in the cervical cord, produced by neck flexion. The symptoms disappear after treatment with vitamin B₁₂. The clinical importance of this symptom is emphasized.