Progressive lumbar kyphosis (camptocormia), a rare, usually familial disease in elderly patients, is characterized by inability to immobilize the lumbar spine in relation to the pelvis. CT scan reveals selective involvement of the spinal muscles with a heterogeneous appearance and is in favour of a primary disorder of these muscles. Our aim was to define the muscular lesions and clarify their nature in this particular disorder. Biopsies of the paravertebral muscles of 14 patients with lumbar kyphosis and of 20 operated on for disc herniation or narrowed lumbar canal, were studied by light microscopy, histochemistry and electron microscopy. In both groups, type 1 fibre predominance and atrophy of type 2 fibres were observed. Ragged-red fibres with abnormal mitochondria also occurred. The differential feature was increased frequency of extensive diffuse or lobulated fibrosis in camptocormia. Other features related to pathogenesis could not be determined.