The frequency of the two tau-fractions in isoelectric focusing examination of cerebrospinal fluid proteins has been studied in 674 neurological patients. The double tau-fraction was found in 151 patients (22.4%); this finding was not characteristically associated with any particular disease, but its frequency was high in hereditary diseases (13 cases out of 30; 43.3%), mainly Charcot-Marie-Tooth's disease.