As many studies around the theme of 'too much medicine' attest, investigations are being ordered with increasing frequency; similarly the threshold for providing treatment has lowered. Our contention is that trust (or lack of it) is a significant factor in influencing this, and that understanding the relationship between trust and investigations and treatments will help clinicians and policymakers ensure ethical decisions are more consistently made. Drawing on the philosophical literature, we investigate the nature of trust in the patient-doctor relationship, arguing that at its core it involves a transfer of discretion. We show that there is substantial empirical support for the idea that more trust will reduce the problem of too much medicine. We then investigate ways in which trust can be built, concentrating on issues of questioning, of acknowledging uncertainty and of shouldering responsibility for it. We argue that offering investigations or treatments as a way of generating trust may itself be an untrustworthy way of proceeding, and that healthcare systems should provide the institutional support for facilitating continuity, questioning and the entrusting of uncertainty.