Seven patients with large, pendulous breasts received external beam irradiation to the whole breast (50 Gy in 25 fractions plus boost) after lumpectomy. A special breast holding mask was shaped for each patient. Three-dimensional treatment plans and dose-volume histograms of the same patient with and without a mask were compared. Day-to-day variance was checked. This technique provides improvements in dose homogeneity, reproducibility, and sparing of normal tissues. Cosmetic outcome at present is good.