BACKGROUND: Although light therapy has become the accepted treatment for patients suffering from seasonal affective disorder (SAD, winter depression), almost 40% of these patients do not respond, and require an alternative treatment. METHODS: The therapeutic effects of light versus tryptophan on SAD were studied in a repeated measures design in 13 SAD patients (11 women, 2 men). Light therapy for 2 weeks or tryptophan for 4 weeks was given, separated by a one week washout period. All were assessed with the modified Hamilton Depression Rating scale (SIGH-SAD) at the beginning and end of each treatment. RESULTS: Four (31%) of the patients did not respond to either therapy. Four tryptophan-resistant patients responded to light therapy, while one light therapy-resistant patient responded to tryptophan. Relapse occurred rapidly after stopping light therapy but not after stopping tryptophan therapy. CONCLUSIONS: There were significant therapeutic effects of both light (p = 0.012) and tryptophan (p = 0.014) on SAD, which were not significantly different from each other. There may be a time difference between the residual pharmacokinetic effects after stopping therapy. LIMITATIONS: The groups studied were small. This was an open study. CLINICAL RELEVANCE: Tryptophan was equally effective to light therapy in treating SAD, but relapse after withdrawal of tryptophan probably occurs more slowly.