The myotubular myopathies are a heterogeneous group of muscle disorders in which x-linked, autosomal recessive, and autosomal dominant inheritance have been reported. Female carriers of x-linked myotubular myopathy have been reported to have abnormal muscle biopsies. We report a woman who had a normal muscle biopsy but who had 2 sons with myotubular myopathy by different fathers, indicating that a normal muscle biopsy of the mother cannot exclude x-linked inheritance. The quantity of fetal activity correlated with the severity of the disorder in this pedigree.