The desire to improve local tumour control and cure more cancer patients, coupled with advances in computer technology and linear accelerator design, has spurred the developments of three-dimensional conformal radiotherapy techniques. Optimized treatment plans, aiming to deliver high dose to the target while minimizing dose to the surrounding tissues, can be delivered with multiple fields each with spatially modulated beam intensities or with multiple-slice treatments. The author introduces a new method, intensity-modulated arc therapy (IMAT), for delivering optimized treatment plans to improve the therapeutic ratio. It utilizes continuous gantry motion as in conventional are therapy. Unlike conventional are therapy, the field shape, which is conformed with the multileaf collimator, changes during gantry rotation. Arbitrary two-dimensional beam intensity distributions at different beam angles are delivered with multiple superimposing arcs. A system capable of delivering the IMAT has been implemented. An example is given that illustrates the feasibility of this new method. Advantages of this new technique over tomotherapy and other slice-based delivery schemes are also discussed.