Children with untreated coeliac disease are characteristically unhappy and after a few days of treatment with a gluten-free diet their mood improves. This improvement in mood can be rapidly reversed by introducing gluten into their diet again which suggests that a humoral agent could be involved in this process. As serotonin is a neurotransmitter in the brain and abnormalities of serotonin metabolism have been reported in coeliac disease, this biogenic amine could be the humoral agent that mediates the changes of mood in coeliac disease. In this review the relationship between the mood changes in coeliac disease and serotonin metabolism will be further examined.