We report a second child with broad A band disease. This child differs from the first in having normal vision and no electrophysiological evidence of a congenital retinal dystrophy. Neurological abnormalities at presentation included diffuse hypotonia, developmental delay, and delayed speech development. Histological and preliminary histochemical evaluation of biopsied thigh muscle showed no abnormality. However, 1 -pm-thick plastic sections and electron microscopy showed numerous foci of broadened A bands accompanied by loss of distinct I bands. The Z lines in these areas were normal except for a fine waviness. Ultrastructurally, the thick filaments in these lesions appeared misaligned. lmmunohistochemical reactions for desmin, vimentin, connectin (titin), and 28 myosin showed normal reactivity. An immunohistochemical reaction for fetal myosin showed sparse reacting fibers, which were unremarkable on adjacent sections stained with hematoxylin and eosin. These findings differ from those of other previously described congenital myopathies. Both of our patients have shown good strength and motor development by 5 years of age, suggesting that this ultrastructural abnormality is essentially benign.