In situ hybridisation has been used to study interferon gamma (IFN gamma) mRNA expression in the small intestine of patients with coeliac disease. Sections of jejunal biopsies were obtained from five patients with treated and five with untreated coeliac disease and five disease controls. These sections were hybridised with radiolabelled specific DNA oligonucleotide probes. The lamina propria of untreated coeliac disease patients contained a significantly increased number of IFN gamma producing cells compared with controls but there was no significant difference between the coeliac patients treated with a gluten free diet and controls. The results suggest that IFN gamma may play a part in the immunopathogenesis of coeliac disease.