Blood loss from the gastrointestinal tract was measured in 13 infants with iron deficiency anemia, using radioiron as a tracer. The radioiron was given intravenously; radioactivity in the erythrocytes and feces was measured for the following 3 to 4 weeks. The percentage of total Fe administered that was recovered in the stools varied between 0.75 and 16.4%, with a mean of 5.75%. This represents loss of whole blood in the stools varying from 7 to 107 ml, with a mean of 41 ml during the observation period. Occult blood loss from the gastrointestinal tract appears to be a significant factor in the development of iron deficiency in early childhood.