Breast conservation therapy has become a common modality for therapy of early stage breast cancer. Most studies of primary lung irradiation correlate the risk of pneumonitis with the volume of lung treated. It is proposed that the lung volume treated during tangential radiation of the intact breast may be calculated from a measurement of the central lung distance. Central lung distance is the distance from the chest wall to the edge of the field at the central axis. This study examines whether the incidence of radiation pneumonitis indeed correlates with the lung volume treated as calculated from a measurement of the central lung distance. From January 1, 1985, through December 31, 1998, 353 patients were treated with breast conservation therapy at the University of Louisville. The charts were reviewed for any evidence of clinically significant radiation pneumonitis. The treatment films were obtained and the central lung distance ascertained. Four cases of radiation pneumonitis were identified. Three of these occurred after tangential radiation and one occurred in a retreatment field after bone marrow transplant. The overall rate of radiation pneumonitis was 1.2%. A central lung distance of less than 3 cm was found in 78.3% of patients and 21.7% had a central lung distance of three or greater. There was no correlation between central lung distance and the risk of radiation pneumonitis. Breast conservation therapy is a well-tolerated modality for treatment of early stage breast cancer. The incidence of radiation pneumonitis is very low and is usually associated with other factors in addition to the tangential field lung volume.