The use of exhaled breath condensate (EBC) as a tool for noninvasive assessment of lung inflammation is becoming commonplace. Many authors use commercial ELISA kits to measure inflammatory mediators within EBC. However, the very low concentrations of mediators within EBC are often below the commercially validated concentration range of the relevant ELISA and crucially below the linear part of the sigmoid standard curve. The present study seeks to validate a series of assays for use in EBC and to compare the results in EBC with those from matched sol phase sputum samples. The following mediators were measured by ELISA: leukotriene (LT)B(4), interleukin (IL)-8, secretory leukoprotease inhibitor and alpha(1)-antitrypsin (AAT). Myeloperoxidase was measured by chromogenic substrate assay. Mediator concentrations reached the lower limit of quantification in only one assay (AAT) in 19.6% of subjects, while mediator concentrations reached the lower limit of detection in three assays (LTB(4), IL-8 and AAT in 31, 6.5 and 61% of subjects, respectively). No significant correlations were present between any mediators in EBC and sol phase sputum. The results of the present study indicate that care must be exercised when interpreting mediator measurements in exhaled breath condensate and that assays must be validated at concentrations relevant to those found within the biological fluid.