To define the amount of epithelial lining fluid (ELF) that is recovered during bronchoalveolar lavage (BAL) in the pediatric age group, we measured albumin and urea concentrations in serum and BAL fluid (BALF) of 37 children aged 3-15 years without bronchopulmonary disease. The children were studied while undergoing elective surgery for non-pulmonary illnesses using a BAL protocol adjusted to body weight. ELF increased with age in proportion to increases in lavage volume. When corrected for the child's weight, approximately 20 mu l ELF/kg body weight were recovered throughout the age range. ELF derived by determining albumin or urea concentrations in BALF were significantly correlated; however, considerable variability was observed in older children when the urea method was used. This was likely due to the increase in dwell time that is known to affect urea concentrations in BALF. In children ELF/100 ml BALF was higher than in adults, suggesting a greater permeability of the alveolar membrane in children. These data show that a BAL protocol adjusted to body weight will yield constant fractions of ELF in children aged 3-15 years. These results should facilitate the comparison of BALF constituents in children of different age groups.