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Three pedigrees are described in which 28 living siblings suffered from a benign myopathy. The first symptoms were observed around the fifth year of life. The proximal muscles were more involved than the distal muscles, the extensors more than the flexors. Due to a marked paresis of the extensor digitorum communis muscles 22 patients showed a flexion contracture of the interphalangeal joints of the last four fingers. In addition 20 patients showed a flexion contracture of the elbows and 12 patients had a plantar flexion contracture of the ankles. A high incidence of congenital torticollis was found. The histopathological features were non-specific and remarkably uniform and consisted of a marked variation in muslce fibre diameter and a very marked increase of fatty tissue. Light-microscopy and electronmicroscopy did not show any specific structural changes. There was normal distribution of type I and type II fibres, without type-grouping or preferential atrophy of one of the fibre types. Lobulated type I fibres were found in 6 out of 12 biopsies. Post-mortem study of one case did not show any convincing features of a neurogenic disorder. As no relationship could be found between the siblings of the 3 pedigrees as far back as the beginning of the eighteenth century, this myopathy seemed to be a new nosological entity.

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A second family with X-linked myotubular myopathy is described. The clinical picture includes decreased fetal movements; hydramnios, in at least three cases, probably resulting from insufficient swallowing in utero; and asphyxia at birth. In three autopsy cases many myotubes were found in the muscle tissue. In five definite female carriers, muscle biopsy revealed changes, including myotubes in four. This family probably is not related to the eariler described family with X-linked myotubular myopathy, from which it differs in its 100 percent fatal outcome in the neonatal period, as compared with 25 percent in the eariler described family. A most important finding, in both families, is the possibility of recognizing clinically healthy female carriers by muscle biopsy.