The relative percent dose reduction by lead of 7- to 18-MeV electrons with a Siemens betatron and of 19- to 28-MeV electrons with a Sagittaire linear accelerator has been measured with a thin-wall buildup chamber for 6.3 X 6.3- and 10.5 X 10.5-cm field sizes at the chamber position for the normal treatment source-to-skin distance (SSD) of each machine. The thickness of lead necessary to attenuate the open beam by 95-98% was then determined of 7- to 28-MeV electrons. The required thickness of lead to attenuate 95% of the 7- to 28-MeV electron beam ranged from 2.3 to 18 mm for the 6.3 X 6.3-cm field and from 2.4 to 23 mm for the 10.5 X 10.5-cm field, respectively. For 98% attenuation, thicknesses from 2.6 to 25.0 mm for the 6.3 X 6.3-cm field and from 2.8 to 27.5 mm of lead for the 10.5 X 10.5-cm field were necessary.