A false-positive 111In-labelled leukocyte scan was obtained in a girl with cystic fibrosis and infection. There was focal accumulation of 111In activity in the abdomen and marked pulmonary uptake. Surgical exploration and postmortem examination revealed no abdominal abnormality. The abdomen activity was apparently due to swallowed sputum which contained labelled leukocytes. Broader clinical implications are discussed.