Two groups of patients with folic acid responsive neurological and psychiatric disorders are reported. The first group (7 patients) had well-established acquired folate deficiency due either to defective absorption (4 cases with atrophy of jejunal mucosa) or to a deficient diet (3 cases). One patient had a subacute combined degeneration of the spinal cord while others were depressed and had weight loss, permanent muscular and intellectual fatigue, restless legs syndrome, depressed ankle jerks, diminution of the sense of vibration in the knees and a stocking-type tactile hypoesthesia. The second group (9 patients) comprised idiopathic cases of folic acid deficiency. Their main subjective complaints were chronic fatigability and familial restless legs syndrome. The neurological findings were similar to those of the patients with acquired disorders. Neuropsychological testing procedures revealed an abnormal intellectual functioning in all 16 patients. Abnormal patterns of radionuclide cisternograms and computerized transaxial tomography (CTT) were found in 11 patients. After 6-12 months of folic acid therapy a striking improvement regarding their intellectual functioning was noticed: the IQ, Kohs Block Design and Category tests were significantly improved. The correlation of neuropsychological findings with CTT and radionuclide cisternograms led to the conclusion that chronic folate deficiency could induce cerebral atrophy.