Bronchoalveolar lavage (BAL) fluid is a variable mixture of instilled and lung fluid, which makes interpretation of solute concentrations difficult. We describe the use of inulin as a marker of dilution of BAL in human subjects. BAL, using saline containing 0.1 mM inulin, was safely performed in 13 subjects with mild asthma and 11 normal subjects. The dilution factor (DF: inulin concentration in BAL fluid/inulin concentration in instilled fluid) was measured spectrophotometrically, and it was used to calculate the volume of lung fluid in BAL fluid. There was no significant difference between the median (range) DF of 0.931 (0.825 to 0.952) in asthmatics and 0.907 (0.768 to 0.985) in control subjects (p = 0.77). There was wide individual variation in, but no significant difference between, the lung fluid volume of 8.1 ml (5.4 to 22.2) in asthmatics and 12.3 ml (1.9 to 30.6) in control subjects (p = 0.56), thus validating comparisons of concentrations per ml of BAL fluid. Alternatively, concentrations can be compared per ml of lung fluid. Inulin fulfilled the requirements for a marker of dilution of BAL, enabling the validation and standardization of comparisons of solute concentrations in BAL fluid.