Aims: The paper explores the factors that influence patient involvement in treatment decision-making. Methods: An observation study of 49 new consultations between oncologists and recently diagnosed bowel cancer patients was conducted at a regional cancer centre in the South West of England. The dialogue was recorded verbatim during each consultation and transcribed. The data were coded into relevant themes and analysed using the constant comparison method. Results: A significant minority (23 out of 49) of patients were involved to varying degrees in decision-making. 'Active' involvement usually resulted from: (1) conflicting expectations between the doctor and the patient about the most appropriate treatment; (2) unexpected information; (3) issues related to treatment costs and benefits and (4) the lack of a clear treatment recommendation from the oncologist. Conclusion: The informed consent procedure requests that full information is provided to patients about their condition and its treatment, although there is no parallel requirement to offer an interpretation of the information. In order to help patients make optimal decisions regarding treatment, oncologists need to provide an adequate level of information as well as interpretation to patients about the pros and cons of treatment. Copyright (C) 2004 John Wiley Sons, Ltd.