To test the hypothesis that cyanocobalamin (vitamin B12) is an effective treatment for winter seasonal affective disorder (SAD). 2 weeks of double-blind placebo washout, followed by random assignment to parallel treatments for 2 weeks with cyanocobalamin vs. placebo. Observations were made during weekly outpatient visits. All subjects met criteria for SAD. 27 patients were studied. After the washout period, 14 were randomly assigned to 1.5 mg cyanocobalamin (3 x/day) and 13 remained on placebo on the same schedule. 29 item SIGH-SAD scores were used to determine antidepressant efficacy. No significant differences were found in the responses between the two groups. Cyanocobalamin does not appear to be an effective short-term treatment for depression in SAD patients. The usefulness as a treatment for SAD of the methylated form of Vitamin B12, which has been used extensively in related studies, remains to be explored.