Lesions in the oral cavity are often treated with two opposed lateral fields. These include a significant amount of normal healthy tissue whose radiation tolerance is dose-limiting. The tumor dose can be boosted to tumorcidal levels by brachytherapy or by small electron fields directed straight on the lesion. We have developed a simple attachment to the standard electron applicator of the Varian Clinac 1800 that allows irradiation of small electron fields through acrylic tubes-the oral cones. These tubes have been evaluated in terms of depth dose and field profiles for 6, 9, 12, 16, and 20 MeV electrons using film for relative dosimetry. At these small field sizes there are significant changes in output factors, in the depth dose as well as in the effective size of the field, and a thorough dosimetric evaluation is imperative prior to treatment. The attachment can be manufactured locally at low cost. For reasons of patient safety the assembly is collapsible. In clinical practice the cone is directed directly on the tumor. For deep-seated lesions we use a penlight and a mirror for positioning.