The case histories of four patients with myotonia congenita are presented in an effort to acquaint the orthopaedic surgeon with this rare disease and to illustrate the difficulties attendant upon its diagnosis. The diagnosis is difficult, not only because the illness is so uncommon, but also because it may not conform to the dramatic description advanced by Thomsen of his own illness and quoted by most current texts that describe the disease. Careful physical examination, which should include an attempt to elicit the myotonic reaction, supplemented by electromyography and serum enzyme studies, should allow accurate diagnosis.