Because it has long been recognized that the growth of most untreated tumors is well described by Gompertzian or exponential growth curves but that the growth of treated tumors has never been well characterized mathematically, we developed and applied an equation that, while not dependent on restrictive assumptions or unmeasurable variables, was nevertheless capable of describing perturbed as well as unperturbed growth. For the 3964A rat hepatoma, a functional relationship between dose level of radiotherapy and growth-curve response was derived and used for predictive purposes. An implied relationship between tumor size and sensitivity to therapy and some clinical implications were examined.