Three familial varieties of periodic paralysis have been described: 1. hypokalemia, 2. hyperkalemic and 3. normokalemic (1, 2). In some eases this classification is not applicable, since there is not always a relationship between the onset or degree of paralysis and the level of serum potassium (1, 3). The fundamental defect causing the periodic attacks of paralysis and the permanent weakness which is sometimes associated is not known. Among the factors advanced as explanations are alteration of the sarcolemma, enzymatic disorder, central nervous system dysfunction, defect of the endoplasmic reticulum, and abnormal carbohydrate metabolism (1, 3–5). Recently, we evaluated a patient with familial hypokalemic periodic paralysis associated with permanent weakness and wasting of the limb musculature proximally. Some of our clinical and ultrastructural findings differ from those previously recorded. By reporting additional information about this illness, we believe progress can be made toward a better understanding of the disease process.