Radiotherapy treatments for early stage breast cancer patients potentially affect the lung in its most distal air spaces, and previous studies have indicated consistently low baseline values for diffusing capacity in breast cancer patients. We aimed to quantitatively assess baseline small airway function and the acute effects of radiotherapy in breast cancer patients with no confounding effects from respiratory disease or considerable smoking history. In 60 breast cancer patients selected from an ongoing randomized controlled trial, the small airways function was assessed at baseline and 3 months later, after having received either conventional radiotherapy (CR; n = 26) or hypofractionated tomotherapy (TT; n = 34). All indices of small airway function in breast cancer patients were found to be indistinguishable from healthy controls. The total lung capacity was significantly decreased and ventilation heterogeneity was significantly increased 3 months after baseline in the CR arm, but not in the TT arm. When corrected for hemoglobin and lung volume, pulmonary diffusing capacity was not affected by radiotherapy in either treatment arm. Alternatively, discarding patients receiving chemotherapy or loco-regional treatment did not affect these results. We conclude that middle-aged women with breast cancer, but no history of respiratory disease, have normal baseline small airways function. Conventional radiotherapy induces a restrictive pattern and increases heterogeneity of ventilation, the latter most likely resulting from differential expansion between locally irradiated peripheral lung zones and the remainder of the lung. The TT modality did not lead to any such changes.