The progress of infant lung function testing has been retarded by both the lack of user-friendly, widely available and affordable equipment and the lack of standardized methodology. The European Respiratory Society/American Thoracic Society Task Force on Standards for Infant Respiratory Function Testing was formed in an attempt to address these deficiencies. This document represents the consensus of investigators with vast experience in the measurement of lung function in infants, The present recommendations deal with equipment requirements, study procedures and reporting of data for measurements of forced expiration at end-tidal inspiration, They represent the "state of the art" in 1999. They are not meant to inhibit further developments in this technique. The authors anticipate that these guidelines will be updated regularly as knowledge progresses.