We studied two patient populations, 9 children with cystic fibrosis and 5 premature infants, in a limited phase I study to determine whether dysprosium administered in trace amounts could serve as a suitable nonabsorbable gastrointestinal marker. Quantitative recovery of dysprosium would allow us to devise a new, rapid and simple approach to measure fecal fat absorption. This technique would be clinically important in the study population. We were able to recover 104.3+/-8.9% of the dysprosium in the cystic fibrosis patients and 99+/-33% of the dysprosium in the premature infants. On the basis of the phase I results in the cystic fibrosis patients, we have submitted a phase II proposal.