Rising costs of cancer care and the growing burden of cancer in a world of finite resources seem to make rationing in oncology inevitable. Information is currently lacking about oncologists' strategies in responding to resource constraints and the prevalence of withholding costly treatments. An online survey was offered via e-mail to physician members of the German Society of Hematology and Oncology. Those actively practicing were asked to complete an online questionnaire asking how limited resources were currently affecting their clinical practice. Two-thirds of 345 participating oncologists reported withholding costly treatments in at least some instances. Regarding their rationale, 70% stated that evidence for costly intervention was not convincing enough, and 59% said that they rationed approved treatments because of an unfavorable cost/benefit calculation. Only 29% reported being explicit about their rationing decision if the patient did not know or inquire about the respective intervention. Withholding expensive procedures from individual patients was widespread among the respondents. Oncologists withheld treatments not only if they perceived the scientific evidence to be questionable but also if they perceived reimbursement prospects or the cost/benefit ratio to be unfavorable, a behavior that could be called rationing. Currently this mostly refers to costly procedures with limited additional benefits. Although this result may be interpreted as indicating that oncologists assume responsibility for spending the resources in a justified way, more transparency and an open discussion on cost-effectiveness and the just allocation of costly treatments is needed.