BACKGROUND: Many decisions can be understood in terms of actors' valuations of benefits and costs. The article investigates whether this is also true of patient medical decision making. It aims to investigate (i) the importance patients attach to various reasons for and against nine medical decisions; (ii) how well the importance attached to benefits and costs predicts action or inaction; and (iii) how such valuations are related to decision confidence. METHODS: In a national random digit dial telephone survey of U.S. adults, patients rated the importance of various reasons for and against medical decisions they had made or talked to a health-care provider about during the past 2 years. Participants were 2575 English-speaking adults age 40 and older. Data were analysed by means of logistic regressions predicting action/inaction and linear regressions predicting confidence. RESULTS: Aggregating individual reasons into those that may be regarded as benefits and those that may be regarded as costs, and weighting them by their importance to the patient, shows the expected relationship to action. Perceived benefits and costs are also significantly related to the confidence patients report about their decision. CONCLUSION: The factors patients say are important in their medical decisions reflect a subjective weighing of benefits and costs and predict action/inaction although they do not necessarily indicate that patients are well informed. The greater the difference between the importance attached to benefits and costs, the greater patients' confidence in their decision.