Although most clinicians agree that patients should be informed about treatment alternatives, little is known about the way patients perceive probabilistic information about treatment outcomes and how it influences the choices they make. The purpose of this study was to examine the influence of level and framing of probability on preferences for cancer treatment alternatives in which tradeoffs between quantity and quality of life are made. 129 healthy volunteers and 154 cancer patients indicated their preferences for a toxic treatment over a non-toxic treatment at varying survival probabilities. Subjects responded to questions in one of three randomly assigned conditions: (1) a positive frame in which the probability of survival was given; (2) a negative frame in which the probability of dying was given; and (3) a mixed frame in which the probability of surviving and dying were both given. The cancer patients' preferences for the more effective toxic treatment was significantly stronger than the healthy volunteers. Both groups were significantly influenced by the level of probability that was presented. Preferences for the toxic treatment were weaker when the chance of survival dropped below 50%. This weakening preference below 50% survival was enhanced for subjects who responded in the negative frame. A negative frame or probability level below 0.5 would seem to stimulate a "dying mode" type of value system in which quality of life becomes more salient in decision making than quantity of life. The implications in eliciting informed consent from patients are discussed.