Bronchoalveolar lavage is a powerful technique for sampling the epithelial lining fluid (ELF) of the lower respiratory tract but also results in a significant dilution of that fluid. To quantify the apparent volume of ELF obtained by bronchoalveolar lavage, urea was used as an endogenous marker of ELF dilution. Since urea diffuses readily through the body, plasma and in situ ELF urea concentrations are identical; thus ELF volume can be calculated using simple dilution principles. Using this approach, we determined that with a standard lavage procedure, the volume of ELF recovered from a normal human is 1.0 +/- 0.1 ml/100 ml of recovered lavage fluid. Time course experiments in which the saline used for lavage was permitted to remain in the lower respiratory tract for various "dwell times" suggested that diffusion of urea from sources other than recovered ELF can contribute to the total urea recovered resulting in an overestimate of the volume of ELF recovered. Thus, while reasonably accurate, the volume of ELF determined by urea must be considered an overestimate, or "apparent" volume. The ELF albumin concentration based on the apparent ELF volume was 3.7 +/- 0.3 mg/ml, a value that is in good agreement with direct measurements made by other techniques in experimental animals. The density of all inflammatory and immune effector cells on the epithelial surface of the lower respiratory tract, based on the apparent ELF volume, was 21,000 +/- 3,000 cells/microliter, a value that is twofold greater than that in blood.(ABSTRACT TRUNCATED AT 250 WORDS)