An adequate radiation treatment plan should give the actual dose distribution at all pertinent points within the patient. Most plans are based on the assumption that the irradiated tissues are of homogeneous unit density. In certain regions, however, there are major deviations from homogeneity and here treatment plans should be corrected for the presence of varying tissue components. The effect of these inhomogeneities may be estimated by making an assumption as to the distribution of bone, muscle, and fat in the region to be treated and calculating the dose configuration with the aid of absorption and scattering coefficients and the isodose distribution in water (1). In practice, however, it is difficult to determine the thickness of each tissue traversed, so that this method is seldom applicable. One method of gross correction for the variation in tissue distribution of patients treated by rotation, or by telecobalt units, has been the use of a transit meter, which measures the primary beam attenuation and relates it to the amount of absorbing material present (2–6). Depth dose curves may then be modified accordingly. Transit meters require a large amount of heavy shielding for collimation, or location at a considerable distance beyond the patient to minimize scattered radiation effects. Versatility is, therefore, limited in most cases to a permanent installation, and measurements are restricted to a single axis, usually the central axis. Another method for making corrections is provided by an exit dosimeter (7). With this instrument, the exit dose on the body of the patient is measured relative to the maximum dose, and the treatment plan is then modified according to these values. The exit dosimeter described here is light and versatile and may be used at any position on the patient, for any direction of field. It is designed to measure the actual exit dose at any specified point, permitting appropriate modification of the planned isodose distribution. This instrument has been calibrated and tested for 250-kv x-rays and for 2-Mev Van de Graaff x-rays. Its use with 2-Mev Van de Graaff x-rays is reported here. Exit Dosimeter Details: The exit dosimeter, as illustrated in Figures 1 and 2, is a parallel-plate ionization chamber with dag-coated polystyrene walls. The upper wall is 1.5 mm. thick and the lower wall 3 mm. thick, with a 1.5 mm. spacing between plates. Guard rings surround both the high-voltage and collector electrodes and are connected to ground, as are the outer exposed surfaces of the dosimeter. The diameter of the collecting electrode is 3 cm., which results in a collecting volume of 1.12 c.c. The chamber electrodes are connected through a 2-foot cable to a preamplifier with an input resistance of 4 × 1010 ohms, and this is connected through a 50-foot cable to a Radocon amplifier, where the dose rate is read directly.