PURPOSE: This study was designed to assess the incidence of pulmonary toxicity (PT) and related risk factors in breast cancer patients treated with postoperative adjuvant radiotherapy (RT) with or without the use of chemotherapy. METHODS: The whole breast or chest wall was irradiated with two tangential photon fields to a total dose of 50.4 Gy in 261 patients. A single anterior oblique photon field for a supraclavicular (SCL) node included if indicated. All patients underwent three-dimensional RT planning (3D RTP), and the calculation of dose volume histogram (DVH) parameters (i.e., ipsilateral lung volume that received > or =15 Gy [V15], V20, V30, and mean lung dose [MLD]). The relationship of symptomatic radiation pneumonitis (SRP) and pulmonary toxicity as discerned by radiographic features (radiographic pulmonary toxicity or RPT) with the clinical and DVH parameters were evaluated. In addition, the relationship of severity of RPT with the DVH parameters was assessed. RESULTS: SRP and RPT developed in 1.9% (5/261) and 22.6% (59/261) of patients, respectively. Age (p=0.008), inclusion of an SCL field (p<0.0001), use of chemotherapy (p<0.0001), use of taxane (p<0.0001), and all DVH parameters (p<0.0001) were associated with RPT in univariate analysis. Based on the results of multivariate analysis, V30 (p<0.001), age (p=0.001) and use of taxane (p=0.036) were significant risk factors in the development of RPT. None of the DVH parameters was associated with the severity of RPT. CONCLUSION: The incidence rate of SRP was very low and there was no correlation between any clinical factor or DVH parameters and SRP. Age, the use of taxane-based chemotherapy and V30 were correlated with the development of RPT.