Clinical studies have been initiated in conformal radiotherapy using a computer controlled multi-leaf collimator. Quantitative dosimetry and treatment planning studies comparing field shaping by lead alloy blocks and the multi-leaf collimator demonstrate the clinical acceptability of the multi-leaf collimator. Sixteen patients with tumors in multiple sites have received some part of their treatments with both blocking systems. Studies of dosimetry and field shaping show that the multi-leaf collimator produces clinically acceptable blocking for most field shapes and disease sites. The 80-20% penumbra was characterized for a wide range of shaped beams. For straight edges perpendicular to the leaf travel, the penumbra of measured dose distributions from the multi-leaf collimator is equal to conventional divergent blocking. When the multi-leaf collimator leaves approach a contour at an angle, the penumbra increases. At forty-five degrees, the maximum angle of approach, the penumbra is approximately 4 mm wider than that for divergent blocks. Three-dimensional treatment planning demonstrates that equivalent dose distributions can be obtained from the two field shaping systems. The multi-leaf collimator can be used effectively and efficiently to treat a variety of disease sites. Its optimal utility may be in treating complex fields--five or more shaped coplanar or non-coplanar beams. It is well suited for conformal therapy applications.