Cancer patient associations report a growing number of members and increasing possibilities to influence health care, but knowledge about the members' views on the benefit of involvement is scarce. We therefore investigated how members (n = 1742) of Swedish patient associations for breast cancer and prostate cancer rate the benefit of membership for their physical and psychological well-being and social adjustment to cancer. Using a scoring scale, 2/3 of the members reported that membership had benefit for psychological well-being, whereas half of the members reported benefit for physical well-being and social adjustment. Individuals who had been actively involved in board work and/or contact person activities within the associations reported significantly more benefit for all three parameters. Gender differences were observed with men, represented by individuals affected by prostate cancer, reporting greater benefit for all three parameters, although especially evident for psychological well-being. Individuals who obtained membership within two years of diagnosis reported greater benefit for psychological well-being and social adjustment compared to those who became members later. In conclusion, members in patient associations for cancer report benefit particularly for their psychological well-being and actively involved members and men affected by prostate cancer perceive the greatest benefit from membership.