Methylation has been implicated in the etiology of psychiatric illness. Parenteral S-adenosylmethionine, a methyl group donor, has been shown to be an effective antidepressant. The authors studied the antidepressant effect of oral S-adenosylmethionine in a randomized, double-blind, placebo-controlled trial for 15 inpatients with major depression. The results suggest that oral S-adenosylmethionine is a safe, effective antidepressant with few side effects and a rapid onset of action. S-Adenosylmethionine induced mania in a patient with no history of mania. S-Adenosylmethionine may be useful for patients who cannot tolerate tricyclic anti-depressants. These findings support a role for methylation in the pathophysiology of depression.