The restless legs syndrome could represent a folate responsive disorder in both patients with acquired-folate deficiency and those with familial symptomatology. Patients with acquired folate-deficiency could be divided into two subgroups. (i) those with minor neurological signs (restless legs syndrome, vibration sense impairment and tactile hypoesthesia in both legs with diminished ankle jerks and a prolonged or assymetrical Achilles-reflex time) and (ii) those with major neurological signs (subacute combined degeneration with or without neuropathies). In some of these patients the classical triad of the malabsorption syndrome is replaced by another triad, constipation, abnormal jejunal biopsy and abnormal d-xylose absorption. A low folic serum acid level could induce minor neuropsychiatric symptoms while an additional low CSF folate could induce major neurological symptoms in spite of the presence of a normal erythrocyte folate level and in the absence of frank anemia. Possible further studies are described.