Complementary and alternative medicine (CAM) is widely used by cancer patients. In order to learn more on the usage of CAM, its reasons and motifs as well as sources of information along the trajectory of treatment, we decided to evaluate the prevalence and predictors for the use of CAM by cancer patients while being under active treatment with chemo- or radiotherapy or in aftercare. We distributed a standardized questionnaire among patients attending a department of radio-oncology, an ambulance for oncology and offices of general practitioners (GPs). Five hundred and six patients took part. Most attributed cancer to stress and trauma (23.7 and 16.4 %) or genes (20.8 %). Forty-four percentage reported knowing a physician with competence in CAM, and in all settings, most patients named the GP. Fifty-one percentage admitted using CAM, 35 % informed the oncologist about using CAM, 56 % informed the GP, and 26 % did not inform any physician. Most often used CAM was vitamin D (17 %) and selenium (16 %). Most important goals were to strengthen the immune system (59 %) and become active (52 %). Most patients were satisfied with the CAM methods they used. Yet, with some methods, dissatisfaction was up to 30 %. The GP has an important function concerning CAM in oncology as most patients believe the GP to have best knowledge in CAM. In order to integrate complementary medicine into evidence-based medicine, physicians should be trained on how to communicate on CAM with the patient and with each other. Explaining cancer and cancer therapies in a way lay persons are able to understand may be helpful. Physicians should actively address patients' needs of involvement not only in decision making, but also actively in the therapy.