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.