This paper provides an overview of two methodologic approaches to investigating patients' treatment preferences. The first approach is derived from formal decision analysis, and is widely advocated for formulating health policy and constructing clinical guide-lines. The author argues that it may not be suitable for application in the context of individuals' decisional behaviour, and then describes the conceptualization, development, and application of an alternative approach to quantifying patients' relative treatment preferences. The approach--the "probability trade-off"--can be structured to illustrate in a highly graphic way the actual complex dilemmas patients often face, and its procedures can be arranged so as to engage the patient in explicitly considering the trade-offs involved. Given these visual and interactive characteristics, and its performance to date, the probability trade-off seems to be a particularly promising technique not only for incorporating patients' preferences into the design of clinical trials, but also for helping patients to comprehend important probabilistic information and then use that information to clarify and communicate their values to the health-care team. The future phases of this cumulative work will be devoted to testing hypotheses that the probability trade-off technique actually can fulfil this function.