Some children with coeliac disease show behavioural disorders such as depression and other signs which have been correlated with reduced central monoamine metabolism. We have therefore investigated the brain availability of the monoamine precursors tryptophan and tyrosine in 15 untreated children with coeliac disease and 12 treated children with coeliac disease as well as in 12 control children. Significantly decreased plasma concentrations of tryptophan were found in untreated children (mean (SD) 13 (4) mumols/l, p less than 0.001) compared with treated children (31 (13) mumols/l), and in both groups of coeliac children when compared with control children (81 (22) mumols/l). A significantly lower ratio of plasma tryptophan to large neutral amino acids (tyrosine, valine, isoleucine, leucine, and phenylalanine) was also observed, which could indicate impaired brain availability of tryptophan in coeliac children and was more pronounced in untreated children. The impaired availability of tryptophan could produce decreased central serotonin synthesis and in turn behaviour disorders in children with coeliac disease.