Myopathy accompanying familial hypokalaemic periodic paralysis (HPP) is much less well documented than the paralytic attacks from which the disease derives its name. Eleven affected members of a large kinship with HPP were studied clinically and radiologically for the presence of myopathy. In 4 patients muscle biopsies were also performed and in 1 of them the histological findings obtained at autopsy were compared with the CT scans of various muscles. In another patient not previously biopsied, the specimens of both amputated legs were examined histologically. The age of the studied individuals ranged from 33 to 74 yrs. The 4 youngest patients showed no clinical signs of myopathy. However, in 2 of them CT scans demonstrated discrete hypodense lesions in the leg muscles, whereas in the other 2, muscle biopsies showed a vacuolar myopathy. The other 7 patients, all older than 50 yrs, presented both clinical and CT evidence of myopathy of proximal and distal muscles ranging from very mild to very severe, males being slightly more affected than females. In all 11 patients a mean CT grading was made that was based on the abnormalities found in the different muscle groups. The myopathy appeared to be unrelated to the history of paralytic attacks, but a strong correlation was found between age and mean CT grading. It was concluded that HPP is a myopathy with permanent muscle weakness of late onset in all the patients. The expression of the paralytic attacks is variable.