Background and Purpose: Radiologic reactions in lung, usually subclinical, are a frequent side effect of radiotherapy for breast cancer. This study was initiated to identify effects of age and tamoxifen on radiation pneumonitis and consequent fibrosis. Patients and Methods: Retrospectively, 451 patients irradiated postoperatively between 1992 and 1995 at the Department of Radiotherapy of Carl-Thiem-Klinikum (Cottbus, Germany) were analyzed. The median age was 58 years. After mastectomy (n = 296), 25 x 2.0 Gy were applied; breast-conserving surgery (n = 155) was followed by 30 x 2.0 Gy. In 221 patients, adjuvant tamoxifen was given. Follow-up included thorax radiography after 15 weeks and 1 year. In patients with reversible observations in standard chest radiography from 15 weeks to 1 year, CT or high-resolution (HR-)CT scans were analyzed after 4-7 years. Results: Clinical symptoms of pneumonitis were seen in 25 patients (5.5%), all with radiologic changes. Early radiologic changes were detected in 134 patients (29.7%). Age (> 58 years; p = 0.0127) and tamoxifen (p = 0.0001) were found as significant parameters of early pneumopathy. Late radiologic changes were seen in 94/425 patients (22.1%), all after a positive early reaction (P = 0.001). Conclusion: A low incidence of clinically symptomatic pneumonitis was observed, while the vast majority of patients presented with early radiologic changes. Higher age and tamoxifen treatment significantly increased the incidence of early pneumopathy. Local fibrotic responses must be expected in all patients with early reactions, indicating a strong consequential component of the late reaction.