Alpha-lactalbumin is a tryptophan-rich protein fraction. A diet enriched with alpha-lactalbumin increases the ratio of tryptophan to the other large neutral amino acids, which may in turn increase brain serotonin content. In stress-vulnerable individuals, alpha-lactalbumin improved mood and attenuated the cortisol response after experimental stress. The aim of the present study was to investigate the effects of an alpha-lactalbumin-enriched diet on mood and stress response in recovered depressed subjects and healthy controls. Forty-three subjects (twenty-three recovered depressed and twenty healthy subjects) received alpha-lactalbumin and casein (placebo) on separate days, in a double-blind randomised crossover design. On both occasions, subjects underwent a stress test (an unsolvable mental arithmetic task with loud noise). The stress test affected mood in both conditions. Although the alpha-lactalbumin diet led to the expected rises in tryptophan and tryptophan:large neutral amino acids ratio, only minimal effects were found on mood and cortisol response to experimental stress. The results were the same for recovered depressed patients and controls. A 1 d diet enriched with alpha-lactalbumin is not sufficient to prevent a stress-induced mood deterioration or a cortisol response in unmedicated, recovered depressed subjects. Future studies may investigate the effects of longer-term diets or may investigate different samples (e.g. medicated patients).