The precautionary principle has often been invoked to justify a ban on genetically modified (GM) crops. However, this justification is based upon a selective application of the principle to the potential public health and environmental benefits of such a ban, while ignoring a ban's potential downside. This is due principally to the fact that the precautionary principle itself provides no guidance on its application in situations where actions (such as a ban on GM crops) could simultaneously lead to uncertain benefits and uncertain costs to public health and the environment. Accordingly, I first develop a framework for applying the principle in cases where the final outcome is ambiguous because both costs and benefits are uncertain. Then, based on a brief survey of the public health and environmental costs and benefits of GM crops, I apply this framework to the broad range of consequences of a ban on GM crops.