OBJECTIVE: We have tested the relapse-preventive effect of citalopram when compared with placebo in 282 patients with Seasonal Affective Disorder (SAD) responding to 1 week of light therapy. METHOD: The response rate to 1-week light therapy and relapse during the continuation phase of 15 weeks were assessed by use of the Hamilton Depression Rating Scale (HAM-D17), the six-item subscale (HAM-D6), the Melancholia Scale (MES), and the combined HAM-D/SIGH-SAD. RESULTS: The response rate to light therapy was 62.5% on the HAM-D17 and the HAM-D6, 56.1% on the HAM-D/SIGH-SAD, 52.8% on the MES. In the continuation phase, citalopram was found superior to placebo on all scales, but the difference was only of statistical significance on the HAM-D6 and the MES. Mean citalopram dose was 26.3 mg. CONCLUSION: Light therapy was found to have and early onset of action. On the HAM-D6 and the MES citalopram significantly reduced the relapse rate in the continuation phase.