Twenty-five patients treated by lumpectomy and radiotherapy for Stage I breast cancer were enrolled in a prospective study to measure the effects of tangential field irradiation on pulmonary function. Fractional doses of 2 Gy to a total of 50 Gy were administered with the tangential technique. An additional 10 Gy (2 Gy x 5) was given as direct booster. Dynamic and static lung volumes, distribution of ventilation and gas transfer were measured before irradiation and at varying intervals up to 1 year after the completion of therapy. There was a small, but statistically significant decrease in the forced vital capacity (mean 63 ml) and the forced expiratory volume in 1 second (mean 79 ml) measured 3 months after irradiation (p less than 0.05). These changes were reversed within 1 year. The reduction in total lung capacity (mean 240 ml) after 3 months was nearly significant (p = 0.06). The remaining variables did not change to a significant degree. We conclude that a slight restrictive ventilatory impairment may occur when a combined tangential and direct booster technique is applied. The changes are, however, small and reversible, and imply no clinical importance.