Abstract A new field flattening filter was designed and constructed for the Varian Clinac-4 Linear Accelerator. Design considerations led to the construction of a conically shaped filter made of 94% lead and 6% antimony. The resultant beam profiles and isodose distributions are compared to those obtained with the brass filter supplied with the accelerator. The new filter resulted in improved dose distributions with reduced penumbra, increased depth dose, and more skin sparing at a cost of 10% reduction in the peak output of the x-ray beam.