Abstract Clinical features as well as light and electron microscopy of muscle are described in 7 children with the Kocher-Debré-Sémélaigne syndrome (KDS) and in 3 children with cretinism but no muscular hypertrophy. Electromyography revealed a myopathic pattern in all 6 tested children with the KDS and was normal in the 1 tested child with cretinism and no muscle hypertrophy. Conventional light microscopy revealed central nucleation, variation in muscle fibre size and shape and abortive spiral annulets. No abnormalities were seen at this level of resolution in the 3 cretins with no clinical evidence of muscular hypertrophy. High resolution light microscopy and electron microscopy revealed abnormalities in muscle of KDS children as well as those without muscular hypertrophy. Electron-microscopic alterations included glycogen and mitocondrial aggregates, dilated sarcoplasmic reticulum profiles, honeycomb configurations of membrane profiles, ringbinden, Z-line irregularities as well as varying degrees of myofilamentous loss and disarray. Neuromuscular junctions were normal. Our findings are contrasted with those in the literature. Experimental evidence, possibly relating some of the observed alterations to the hypothyroid state, are cited from the literature. It is concluded that the myopathology of KDS is variable and nonspecific and that pathological alterations do occur in muscles of cretins without concomitant clinical muscular hypertrophy.