A 73-year-old man with recurrent periodic paralytic episodes lasting for two weeks each admitted to our hospital because of the leg weakness and the elevated value of serum creatine kinase. On admission, weakness in the proximal legs and mild eye lid myotonia were noted. Needle electromyography revealed abundant myotonic discharges. The prolonged exercise test showed a continuous reduction of compound muscle action potentials in the abductor digiti minimi muscle. Direct sequencing of SCN4A in the proband showed a G-to-A alteration at position 4774 that results in a change of 1592(nd) methionine to valine (M1592V). Cosegregation regarding the M1592V mutation and paralytic phenotype in this family was confirmed. Two cardinal features in this family were longer paralytic episodes compared to classical hyperkalemic/normokalemic periodic paralysis and the normal potassium value during the paralytic episodes. This study together with antecedent reports indicates that M1592V mutation shares a much greater clinical diversity ranging from congenital paramyotonia to periodic paralysis with a longer duration.