Patient participation in treatment decisions presupposes well-informed patients. The purpose of this study was to determine Swedish breast cancer patients' information needs and their preferences for participation in treatment decisions. Patients (n = 201) were interviewed on nine categories of information and five patient roles, using paired comparisons. Patients gave priority to facts about disease and treatment (chances of cure, stage of disease, treatment options). A collaborative role in treatment decisions was preferred by 87% of the patients. Most patients (56%) preferred a passive form of collaboration: I prefer that my doctor makes the final decision about my treatment but seriously considers my opinion. Younger and better educated patients tended to prefer a more active role. Many patients wanted to be more active (20%) and some more passive (8%) than they actually were. Patients gave priority to disease-specific information, but this reflected needs other than taking control of treatment decisions.