Cylindrical spirals are unique membranous structures that were detected in skeletal muscle of a mother and one of her two children; all three have percussion myotonia but no evidence of weak or wasted skeletal muscles. Muscle cramps, stiffness, posteffort muscle tightness, myotonic lid lag, and the cylinders appear or progress with age, but the disorder is asymptomatic in the children and only mildly incapacitating in the mother. The cylinders are 8 mu long and 1 mu wide, are composed of spiraling double-laminate membrane resembling myelin, and are derived from abnormal subsarcolemmal tubulovesicular structures that are interpreted as pathological T-tubes.