Cancer patients were questioned about the consent process in a context in which they were routinely requested to donate tumor samples to research. After in-depth interviews of 19 patients, a 12-page questionnaire was designed and mailed to 745 patients who had been recently treated for colorectal cancer, breast cancer, or a hematological malignancy at a French Regional Cancer Center at which an opt-in biobanking system has existed since 2002. The response rate was 77.0% (N = 574). Among responding patients, 349 (60.8%) of the 574 were in favor of a formal and signed consent. Concordance was low (kappa = 0.23) between the number of patients who declared in the survey that they had given consent (213 of 574 [37.1%]) vs the number for whom registered consent had been recorded (267 of 574 [46.5%]). Only 2 (0.3%) of the 574 patients stated that they had signed a refusal, and only 88 (41.3%) of the 213 patients who remembered giving consent understood that their consent for biobanking also covered authorization to use their clinical data. We conclude that the opt-in consent procedure is positively perceived by most patients but should be improved for a better understanding and possibly an even better adherence to the consent process.