The internal mammary area has been reviewed on CBT scans from 46 postmastectomy patients, and chest wall thicknesses computed there from the scan. Thicknesses ranged from 10 to 43 mm and 16 to 70 mm on the operated and nonoperated sides, and were weight-related in 38 modified mastectomy patients. Direct Cobalt and 15 MeV electron portals delivered 97-98% of the dose prescribed at 3 cm to a depth of 35 mm, the maximum average thickness on the operated side, while 12 and 10 MeV electrons gave 10 and 20% less at that depth. At 70 mm, underdosage with Cobalt and 15 MeV was 22% and 72%; insignificant doses were delivered with 10 and 12 MeV electrons. Routine determination of chest wall thickness is recommended in patients planned for electron beam treatment, in obese patients, and in patients treated with tangential photon techniques, to avoid underdosage to the internal mammary area. Internal mammary nodes were not routinely visualized, but abnormalities consistent with enlarged nodes were seen in six patients and gross recurrence in another.