Twenty children with respiratory disease ingested 500 muCi of Tc-99m sulfur colloid orally, and scintigrams of the thorax were obtained to determine whether pulmonary aspiration of gastric contents could be detected. The children ranged in age from 1 mo to 14 yr; 13 were 8 mo of age or younger. Children were studied at 5 min and 4 hr after ingestion of Tc-99m sulfur colloid using a high sensitivity computer oscilloscope to record 100K-count images. Additional images were obtained after the children had slept overnight. Five children (25%) showed definite pulmonary accumulation of activity; four of these also had a barium swallow and three showed either pulmonary aspiration of barium or moderately severe gastroesophageal reflux. Oral ingestion of Tc-99m sulfur colloid provides a noninvasive means for diagnosing pulmonary aspiration under physiologic conditions in infants and children.