A family presenting a hyperkalemic form of periodic paralysis was studied through six generations; 31 individuals were affected, 14 could be examined. Periods of diffuse or localized muscle weakness, lasting one to four days, were associated with the classic adynamic attacks. Myotonic symptoms were discrete. Some patients, in particular the hard manual workers, exhibited a permanent proximal deficiency. Serum potassium level was in the lower normal range but raised during the attacks. EMG showed a polymorphic pattern, suggesting a functional neuro-muscular block. The histological and ultrastructural changes were pleiomorphic: vacuoles, tubular aggregates and fibers were found in a patient with permanent weakness; a pure type II fibre atrophy was noticed in another patient, without any weakness between the attakcs. Acetazolamide treatment gave good results in four cases out of seven.