Breast cancer is the most common cancer in women, and the second most common cause of cancer death after lung cancer. Improvements in the outcome of breast cancer mean that more patients are living longer and are, therefore, at risk of developing a second malignancy. The aim of this review is to present the current understanding of the risk of lung cancer arising in patients previously treated for early stage breast cancer. We review data on the effect of treatment factors (ie, surgery type, radiotherapy technique, and adjuvant chemotherapy) and patient factors (ie, age and smoking) on the risk of developing a subsequent lung cancer. The evidence suggests that older radiotherapy techniques were associated with a substantially increased risk of developing lung cancer in the ipsilateral lung, but there is no clear evidence of an increased risk with modern techniques. Smoking is an important risk factor, and increases the risk of lung cancer in those receiving radiotherapy. Adjuvant chemotherapy is not significantly associated with an increased risk. The risk of developing lung cancer increases with time elapsed since treatment, but any effect of age at treatment is unclear. © 2010 Elsevier Ltd.