Three patients with Hypokalemic Periodic Paralysis (HOPP)-associated progressive interattack muscle weakness, who became unresponsive or worsened by acetazolamide, responded favorably to dichlorophenamide, a more potent carbonic anhydrase inhibitor. Dichlorophenamide in single-blind placebo-controlled trials, considerably improved functional strength in two of the patients and had a moderate but definite effect in the third. Muscle groups graded 4/5 (MRC scale)returned to normal; very weak (0-3/5) atrophic muscles, improved to a minor degree. In one patient with acetazolamide-resistant paralytic attacks, dichlorophenamide also diminished the frequency and severity of the acute attacks. Dichlorophenamide had, in the present study, less effect than acetazolamide in reducing serum HCO3(-) and elevating Cl-. Its effectiveness may be related to the degree of sensitivity of certain HOPP patients to alterations of Cl- and/or HCO3(-) serum levels or to a different action of the drug unrelated to carbonic anhydrase inhibition or acidosis. Dichlorophenamide should be considered as an alternate to acetazolamide in the treatment of patients with HOPP-associated interattack muscle weakness who have become unresponsive or worsened by acetazolamide.