Aim: To understand participants’ actual motivations to enroll in cohort studies with biobanks. Methods: Participants of two such studies currently ongoing in Germany were invited to fill in a questionnaire about their decision to enroll. Results: From the 623 questionnaires completed, contributing to scientific knowledge appeared as a main motive for enrollment, followed by learning about personal health status and receiving personal medical advice. Conclusion: Motivating participation as a way to contribute to the common good rather than as a way to obtain personal benefits appears to be particularly appropriate to ensure participants’ long-term commitment and should therefore be further investigated in this setting.