We describe a case of a 38-year-old female who presented with diarrhoea and abdominal pain 27 days after a second 'top-up' allogeneic marrow infusion for acute myeloid leukaemia (AML) in first remission. A clinical diagnosis of gut graft-versus-host disease (GVHD) was made. Technetium (99mTc)-labelled white cell scanning and intestinal permeability studies using 51Cr-EDTA and 14C-mannitol were undertaken to confirm the diagnosis. The 99mTc white cell scan showed extensive uptake in the small bowel and the urinary excretion of 51Cr-EDTA was increased, the results being consistent with intestinal inflammation and gut GVHD. 99mTc white cell scanning and intestinal permeability studies may assist in the diagnosis of gut GVHD and in assessing its extent and response to treatment.