Hyperkalemic familial periodic paralysis (adynamia episodica hereditaria) is a hereditary disorder manifested by recurrent paralysis with hyperkalemia rather than hypokalemia as seen in classical familial periodic paralysis. A family with 11 affected members in 3 generations was reviewed, and one of the members, a 20 year old male was studied in some detail. Paralysis characteristically occurs during rest after physical exertion but can occur without preceding physical activity. Potassium administration can provoke paralytic attacks, and hunger often intensifies the degree of paralysis. The onset is usually during the first decades of life but can be later. The paralytic attacks are generally mild before puberty, more severe after puberty, and lessen in frequency and severity after age 30. Inheritance is from a single dominant autosomal gene with complete or almost complete penetrance. The serum K rise during paralysis is usually modest, often not above 6.0 mEq/l and parallels the degree of paralysis. Other serum electrolytes are not altered in any definite pattern during an attack. Urinary excretion of K remains unchanged or parallels the serum level. Chlorothiazide, epinephrine, amphetamine, and thyroid have reportedly altered paralytic attacks, but none were found effective in this study. Although the specific biochemical defect(s) is unknown, perhaps the mechanisms which control the transfer of K between the intra and extracellular spaces play the dominant role.