Pyridoxine (vitamin B6) deficiency developed in two young men treated with phenelzine. Alleviation of symptoms possibly associated with this deficiency and correction of subnormal serum B6 levels occurred with the administration of pyridoxine. With the increased use of phenelzine, all physicians should be alert to this potential problem and instruct patients to eat an adequate amount of foods containing B6. If untoward reactions occur during phenelzine treatment, B6 deficiency should be considered.