A double-blind control trial of gluten-free versus a gluten-containing diet was carried out in a ward of maximum security hospital: 24 patients were studied for 14 weeks. Most suffered from psychotic disorders, particularly schizophrenia. Various dimensions of behaviour were rated on the Psychotic In-Patient profile (PIP) at different stages. There were beneficial changes in the whole group of patients between pre-trial and gluten-free period in five dimensions of the PIP, maintained during the gluten challenge period; these changes could be attributed to the attention the patients received. Two patients improved during the gluten-free period and relapsed when the gluten diet was reintroduced.