Among those decisions that may be made by a patient in response to an illness, the authors single out a certain class. contingent investment decisions. They are characterized by the patient's committing him- or herself, on the basis of prognostic counseling, to a certain action or non-action that he or she may regret in retrospect Examples show that, when assessing utilities, the decision analyst runs a risk of handling such investment decisions incorrectly, unless they are made explicit and incorporated into the medical decision process. The anomaly is explained as a violation of the structural rules for decision trees and is also interpreted in terms of "the price of prognostic ignorance," a quantity closely related to the expected utility value of perfect information. Key words: decision theory; physician-patient relations, patient compliance; prognosis; risk-taking, quality of life; utility theory; patients' decisions, decision trees. (Med Decis Making 1992;12:39-43)