The likelihood of radiation pneumonitis and factors associated with its development in breast cancer patients treated with conservative surgery and radiation therapy have not been well established. To assess these, we retrospectively reviewed 1624 patients treated between 1968 and 1985. Median follow-up for patients without local or distant failure was 77 months. Patients were treated with either tangential fields alone (n = 508) or tangents with a third field to the supraclavicular (SC) or SC-axillary (AX) region (n = 1116). Lung volume treated in the tangential fields was generally limited by keeping the perpendicular distance (demagnified) at the isocenter from the deep field edges to the posterior chest wall (CLD) to 3 cm or less. Seventeen patients with radiation pneumonitis were identified (1.0%). Radiation pneumonitis was diagnosed when patients presented with cough (15/17, 88%), fever (9/17, 53%), and/or dyspnea (6/17, 35%) and radiographic changes (17/17) following completion of RT. Radiographic infiltrates corresponded to treatment portals in all patients, and in 12 of the 17 patients, returned to baseline within 1-12 months. Five patients had permanent scarring on chest X ray. No patient had late or persistent pulmonary symptoms. The incidence of radiation pneumonitis was correlated with the combined use of chemotherapy (CT) and a third field. Three percent (11/328) of patients treated with a 3-field technique who received chemotherapy developed radiation pneumonitis compared to 0.5% (6 of 1296) for all other patients (p = 0.0001). When patients treated with a 3-field technique received chemotherapy concurrently with radiation therapy, the incidence of radiation pneumonitis was 8.8% (8/92) compared with 1.3% (3/236) for those who received sequential chemotherapy and radiation therapy (p = 0.002). A case:control analysis was performed to determine if the volume of lung irradiated (as determined using central lung distance [CLD]) was related to the risk of developing radiation pneumonitis. Three control patients were matched to each case of radiation pneumonitis based on age, side of lesion, chemotherapy (including sequencing), use of a third field, and year treated. Lung volumes were similar in the radiation pneumonitis cases and controls. We conclude that radiation pneumonitis following conservative surgery and radiation therapy for breast cancer is a rare complication, and that it is more likely to occur in patients treated with both a 3-field technique and chemotherapy (particularly given concurrently with radiation therapy). Over the limited range of volumes treated, lung volume was not associated with an increased risk of radiation pneumonitis.