The knowledge of patients about their disease is a precondition for actively participating in decisions on treatment and aftercare. Self-help groups consider themselves as important intermediate agents for conveying disease-related information, but the health-promoting effects of such groups have not yet been studied in much detail. Using a newly developed self-administered questionnaire it was examined whether breast cancer patients involved in self-help groups differ from non-members with respect to their level of knowledge on disease, prevention and after-care. Moreover we considered the influence of age and education on the knowledge on disease. 216 women completed the questionnaire in the presence of an interviewer. Members of self-help groups had a higher level of knowledge than non-members, and this refers to all aspects covered by the questionnaire. Younger women in general had a higher knowledge than older ones, the same holds for higher educational levels. The duration of the disease and the length of membership in self-help groups had no effects on levels of knowledge. Particularly middle-aged women (45 - 59 yrs.) and women with 10 years of schooling profited from a membership. After controlling for education and membership in self-help groups still had a significant impact on knowledge. Since only a minority of patients is willing to be engaged in self-help groups, more effort should be devoted to conveying information in post-treatment care.