The aims of this study was to estimate and describe the reasons of use of complementary medicine in patients with a cancer treated in a French oncology department. An anonymous questionnaire was proposed to patients during treatment or follow-up in an oncology department. It was a descriptive study over 207 questionnaires. Over 195 analysable questionnaires, 34% of patients were using complementary medicine. Homeopathy (42%), plants (27%) and vitamins (18%) were the main substances used. Acupuncture (22%) and massages (15%) were the most significant techniques. No specific profile of user was observed. The main reason of using complementary medicine was not curing cancer but reducing side effects of the conventional treatment (66%). More than a half of the users of complementary medicine were not revealing their habits to their oncologist because the question was not raised in consultation. One third of cancer patients are using complementary medicine during the treatment of their disease. Complementary medicine must benefit, as well as conventional medicine, of scientific studies. The purpose of these studies should be evaluating potential benefits, toxicity and interactions with the conventional treatment in order the oncologist could warned the users. This warning could improve the confidence relationship between the oncologist and his patient.