This report describes a revision of the model used in ICRP Publication 30 to calculate radiation doses to the respiratory tract of workers resulting from the intake of airborne radionuclides. This revision was motivated by the availability of increased knowledge of the anatomy and physiology of the respiratory tract and of the deposition, clearance, and biological effects of inhaled radioactive particles, and by greatly expanded dosimetry requirements. To meet fully the needs of radiation protection, a dosimetric model for the respiratory tract should: provide calculations of doses for individual members of the populations of all ethnic groups, in addition to workers; be useful for predictive and assessment purposes as well as for deriving limits on intakes; account for the influence of smoking, air pollutants, and respiratory tract diseases; provide for estimates of respiratory tract tissue doses from bioassay data; and be equally applicable to radioactive gases as well as to particles. Addressing all of these requirements has resulted in a dosimetry model that is more complex than previous models.