In summary, there is a growing body of clinical evidence along with laboratory studies that in rapidly growing tumors, small doses per fraction given over a short time interval, as in hyperfractionation or accelerated fractionation, may increase the therapeutic ratio. For slowly growing tumors, small doses per fraction may still have an advantage, but they need not be compressed into a short time interval. Finally, for certain "radioresistant" tumors, with a large shoulder to the survival curve, there may be a clinical advantage to larger doses per fraction, but this will have to be weighed against the potential long-term chronic toxicity which increases rapidly with large doses per fraction.