Five generations of a family with hypokalemic periodic paralysis (HOPP) were studied. Of the 120 screened family members, 64 were found to have HOPP of which 38 were suffering from attacks. In the other 26 the diagnosis was made on account of vacuolar myopathy, a reduced muscle fiber conduction velocity or the signs of permanent muscle weakness (PMW) in combination with (grand)children with attacks. Applying these criteria skipping of generations did not occur in this family. When defined properly, PMW was found in all patients at older age, independent of the previous occurrence of paralytic attacks. Dynamometry and muscle CT-scanning appeared valuable in the diagnosis and the progression of PMW. In 2 patients autopsy was performed. Characteristics vacuolization was found in the striated muscle tissue in various degrees. Heart and smooth muscle tissue were not involved. Therapy is limited. Potassium salts shortening and preventing the paralytic attacks are tolerated well. Acetazolamide is more effective in the prevention of the paralytic attacks, but is not tolerated very well. HOPP can be considered as a myopathy characterized by PMW at older age in all patients, combined with paralytic attacks in more than half the patients.