Vitamin B(12) and folate deficiency is often associated with affective disorders mainly of the depressive type. We report a case of a 42-year-old woman with a mood disorder with mixed depressed/manic features that was due to vitamin B(12) and folate deficiency. The psychopathology developed over a five-year period without hematologic or other overt clinical characteristics of pernicious anemia. Replacement treatment with vitamin B(12) and folate was rapidly followed by full clinical remission, electroencephalographic normalization and neuropsychological improvement. At a one-year follow-up this condition was stable. Consequently, patients who respond poorly to psychopharmacologic treatment and/or present with atypical mood symptoms would warrant determination of vitamin B(12) and folate serum levels.