Previous research on moral dilemmas has mainly focused on decisions made under conditions of probabilistic certainty. We investigated the impact of uncertainty on the preference for action (killing one individual to save five people) and inaction (saving one but allowing five people to die) in moral dilemmas. We reported two experimental studies that varied the framing (gain vs loss), levels of risk (probability of gain and loss) and levels of ambiguity (imprecise probability information) in the choice to save five individuals (...) by sacrificing one. We found that participants preferred actions with uncertainty (risk/ambiguity) over inaction. Specifically, we found that participants preferred actions with precise probability information (risk) over inaction, and they preferred actions with modest or high levels of ambiguity over actions with precise probabilities, especially when moral dilemmas had a loss frame. We also observed commission bias in Study 2. We discussed the implications for research in moral decision-making. (shrink)