The use of a multileaf collimator in the dynamic mode to perform intensity modulated radiotherapy became a reality at our institution in 1995. Unlike treatment with static fields using a multileaf collimator, there are significant dosimetric issues which must be assessed before dynamic therapy can be implemented. We have performed a series of calculations and measurements to quantify head scatter for small fields, collimator transmission, and the transmission through rounded leaf ends. If not accounted for, these factors affect the delivered dose to the prostate by 5%-20% for a typical plan. Data obtained with ion chambers and radiographic film are presented for both 6 and 15 MV x-ray beams. The impact on the delivered dose of the mechanical accuracy of the multileaf collimator, achieved during leaf position calibration and maintained during dose delivery, is also discussed.