Surface EMG, muscle fibre conduction velocity (MFCV), muscle force, and biochemical variables were investigated in a 13-year-old boy with familial hypokalaemic periodic paralysis during and after three attacks of paralysis. After normalisation of the serum potassium values, strength rapidly returned to interictal values, but the integrated EMG and to a lesser extent the MFCV recovered more slowly. These findings suggest that a complete electrophysiological recovery is not necessary for a restoration of muscle force and that the pathogenetic defect is localised in the muscle membrane.