Clinical and pathological findings of two patients, a 44-year-old male and a 54-year-old female, with adult-onset nemaline myopathy were described. Both patients showed normal motor development through their childhood; Patient 1 ran fast and was involved in powerful heavy labor until the age of 40, and Patient 2 was in good health until 49, when they began to have progressive muscle weakness. They had no family history of neuromuscular diseases. On neurological examination, they had moderate muscle weakness and atrophy in their limb-girdle and paravertebral muscles. Because paravertebral and neck muscles were preferentially involved, they had difficulty in holding the head straight up. They stood in a lordotic posture. They had neither high-arched palate nor facio-skeletal abnormalities which were common findings in congenital nemaline myopathy. Serum enzymes derived from muscle were normal and needle electromyography showed myogenic and neurogenic changes in both patients. On CT scan of the skeletal muscles, the paravertebral muscles were markedly decreased in density suggesting advanced fat tissue replacement in large areas. In patient 2 who was in more advanced stage, the quadriceps femoris, hamstrings and soleus muscles also showed the similar CT findings. Light microscopic examination of biopsied biceps brachii (Patient 1) and quadriceps femoris (Patient 2) demonstrated abundant rod-like structures in the majority of type 1 fibers. In both patients, there was a marked variation in fiber size. Type 1 fibers were atrophic and type 2C fibers increased in number.(ABSTRACT TRUNCATED AT 250 WORDS)