In 12 patients with paramyotonia congenita, percutaneous needle biopsies from the brachial biceps muscle were performed. Muscle fibre area, distribution of muscle fibre types I, II-A and II-B and capillarization were not different from healthy controls. Signs of myopathy with central nuclei in the muscle cells were noted in 9 of the patients. 4 of these patients also had small areas with degeneration and, in one, vacuoles were observed. Quantitative determination of muscle glycogen, water and protein content were within normal range as were enzyme activities for hexokinase, lactate dehydrogenase, citrate synthetase and 3-hydroxy-acyl-CoA dehydrogenase.