Patients with seasonal affective disorder (SAD) do not necessarily experience depressive episodes every winter. We assessed whether the behavioural response to tryptophan depletion in summer when patients are fully remitted and off therapy is capable of predicting a future depressive episode of SAD. In a prospective study design, we followed up 11 consenting SAD patients who had undergone tryptophan depletion during summer. We evaluated how many of these patients would develop a depressive episode in the subsequent fall/winter. Seven out of eight patients who relapsed during tryptophan depletion in summer developed a depressive episode in the subsequent winter. Two out of the three patients who did not relapse during tryptophan depletion remained well during the follow-up period. Our preliminary findings suggest that those SAD patients who develop depressive symptoms during tryptophan depletion when they are fully remitted and off therapy remain at high risk to experience a depressive episode of SAD also in the subsequent winter.