Three sisters with myopathy characterized by different degrees of weakness, hypotonia, cramps and a significant hypertrophy of the calves underwent clinical tests. Laboratory examinations (nerve conduction velocity, electromyography and serum enzymes), serial histochemical analyses of muscle specimens and tests for muscular acetylcholinesterase (AChE) activity and its molecular forms were performed. AChE activities did not differ significantly from those of controls, while sedimentation patterns evidenced the disappearance of 16 S, 13 S and 10 S molecular forms in the elder sisters. The genealogical tree of the patients is described and their cases compared to those of others with calf hypertrophy reported in the literature.