Serum folate and vitamin B12 levels have been measured in 101 patients with depressive illness. Subnormal folate levels (2.5 ng./ml. or less) were found in 24 per cent and subnormal vitamin B12 levels (150 pg./ml. or less) in 2.2 per cent of patients. Thirteen patients(14.4 per cent) had low vitamin B12 levels (200 pg./ml. or less). Patients with subnormal folate levels were found to have significantly higher depressive scores, and significantly lower validity scores on the Marke-Nyman Temperament Scale, both on admission and on discharge. Subnormal folate levels could be a consequence of dietary deficiency or could possibly be causal of depression through interference with tyrosine or tryptophan hydroxylation.