Quantitative measurements of the effects of irradiation on normal tissues in humans have been hard to obtain because most tissues are inaccessible and/or direct responses are difficult to quantify in a nondestructive manner. Pneumonitis and fibrotic lung disease are adverse effects seen in varying intensity in patients treated with radiotherapy for carcinomas of the thorax, e.g., breast cancer. In the present study the aim was to evaluate the inflammatory reaction in the underlying parenchyma following postoperative irradiation with bronchoalveolar lavage technique. Twenty-one patients (11 smokers, 10 nonsmokers) with breast cancer stage T1N0M0 received radiotherapy with photons to a target dose of 56 Gy following breast conservative surgery. Nineteen healthy controls were also included. The results showed a clear elevation of neutrophils, mast cells, eosinophils, and lymphocytes in the total irradiated group, compared to controls. When subclassifying the material according to smoking habit, it was obvious that the smokers displayed a significantly decreased inflammatory reaction, i.e., reduced levels of mast cells and lymphocytes, compared to both nonsmoking controls and patients. Eosinophils were seen in an elevated number in all irradiated patients. Radiological signs of pneumonitis were observed in three patients, all in the nonsmoking group. No correlation was found between the volume of lung irradiated and the inflammatory response. It is concluded that bronchoalveolar lavage is a suitable and sensitive method for investigating radiotherapy-induced reactions in the human lung. Furthermore, ongoing smoking during the treatment depressed the inflammatory response in the lung parenchyma induced by irradiation. The present study as well as earlier observations justify further studies concerning the possibility of interaction of smoking with cancer treatment, both from the view of therapeutic failures and reduced adverse effects.