Background: According to concerns about the amount of lung irradiated in breast cancer patients treated with Tangential fields, and due to lack of any information about its status in Iran, this study has been carried out to determine the percentage of irradiated lung volume, in breast cancer patients referring to Imam Hossein Hospital during years 2002-3. Materials and methods: Cross Sectional study was performed in 50 women. The percent of irradiated lung volume was calculated, using Tangential simulation. Presumed values greater than 20% were considered as unacceptable. The correlation of apparent chest wall size and also wide surgical incision or extensive remaining breast tissue (within Tangential fields) was also assessed with the above-mentioned volume. Results: Mean age of studied patients was 44±9.1 years, without routine simulation control, in 72% of patients percent age of lung irradiated was unacceptable. The Pearson Correlation between Tangential field length, sternum length, hemi thorax width, hemi thorax thickness and multiplied of 3 later parameters, were: 0.1 (NS), -0.1 (NS), -0.4(NS), -0.11(NS), -0.1(NS) respectively. %78.6 of patients with wide surgical incision or extensive breast tissue and %63.6 without this status had unacceptable irradiated lung volume (NS). Conclusion: Without routine simulation, percent of irradiated lung volume in Tangential fields radiotherapy is unacceptably high, and it seems that apparent parameters of chest wall and also surgical incision or remaining breast tissue, solely and without simulation, are not useful for estimating irradiated lung volume. Therefore, pretreatment routine simulation control is recommended.