PURPOSE: This study investigates the incidence of short-term pulmonary complications following radiotherapy (RT) for breast cancer (BC) with different treatment techniques/incidentally irradiated lung volumes and the importance of confounding factors on RT-induced pulmonary complications. PATIENTS AND METHODS: Prospectively, 475 patients with BC were followed for pulmonary complications 1, 4 and 7 months post-RT. Mean lung dose volume histograms (MDVH) were constructed and compared for the different RT-techniques. Among a subset of the mastectomized patients treated with loco-regional (LR-) RT, who had undergone complete three-dimensional (3-D) dose planning (n = 43), MDVH for asymptomatic patients was compared with MDVH for patients experiencing both radiological and clinical pulmonary side-effects. RESULTS: Moderate pulmonary complications, that is requiring treatment with corticosteroids, were rare following local RT (< 1%), but were diagnosed among 11% of the patients treated with LR-RT. A correlation between increasing irradiated lung volumes at the >20 Gy-level (V20), based on MDVH for the RT-techniques, and pulmonary complications was found (P < 0.001). Furthermore, increasing age and reduced pre-RT functional level were independently associated with a higher rate of pulmonary complications (P = 0.005 and P = 0.018). Among the subgroup of mastectomized patients treated with LR-RT, who had undergone complete 3-D dose planning, a difference in mean V20 was found between patients experiencing both clinical and radiological pulmonary side-effects compared to patients experiencing neither of the two side-effects (P = 0.007). CONCLUSION: Moderate pulmonary complications following local RT for BC are rare. The incidence of short-term moderate pulmonary complications in LR-RT is, however, clinically significant and to define quality assurance guidelines for these RT-techniques, 3-D RT planning can be used.