Cancer screening programmes cause harm to individuals via overdiagnosis and overtreatment, even where they confer population-level benefit. Screening thus appears to violate the principle of non-maleficence, since it entails medically unnecessary harm to individuals. Can consent to screening programmes negate the moral significance of this harm? In therapeutic medical contexts, consent is used as a means of rendering medical harm morally permissible. However, in this paper, I argue that it is unclear that the model of consent used within therapeutic medicine can be applied unproblematically to preventive medicine. Invitation to screening changes the pragmatic norms and expectations of the patient-doctor encounter such that two key principles of consent may be violated. First, the pragmatics of a medical invitation are such that patients may fail to be adequately informed, since patients appear to assume medical invitations are made with their best interests in mind, even where information to the contrary is outlined. Second, screening invitations may place pressure on patients; in the context of a medical encounter, to make an invitation to screening may constitute an inducement to accept. In order to be sure that a patient's consent to a screening invitation is valid, we must make clear to patients that their decision to accept screening may be shaped not only by how information about screening is presented, but by the pragmatic form of the invitation itself.