The authors studied 8 patients with Familial Periodic Paralysis on the clinical, hydroelectrolytic, electrophysiologic, histologic and therapeutic points of view. There was significant predominance in males. The onset of the symptoms in our group began under fifteen years of age. The clinical manifestations were similar to those referred in the literature. The hypokalemic form was found in all patients except in one case that had hyperkalemic form. Natremias on the superior border of the normality, were seen in 21.4% of the cases, and in 21.4% the natremias were higher; this fact suggested aldosterone liberation of the muscle. The histologic examination performed in 5 patients, was normal in 20% of them and, in those cases with repetitive and frequent crises, atrophic muscle fibers were seen. Prophylactic treatments were made. The patients with hypokalemic form received supplementary doses of potassium.