The ability of patients in many parts of the world to benefit from transplantation is limited by growing shortages of transplantable organs. The choice architecture of donation systems is said to play a pivotal role in explaining this gap. In this paper we examine the question how different defaults affect the decision to register as organ donor. Three defaults in organ donation systems are compared: mandated choice, presumed consent and explicit consent. Hypothetical choices from a national survey of 2069 respondents in May 2011 in the Netherlands – a country with an explicit consent system – suggests that mandated choice and presumed consent are more effective at generating registered donors than explicit consent.