OBJECTIVE To develop and test a measure for assessing peer support for men attending prostate cancer support groups, and to describe socio-demographic, medical and adjustment characteristics of Australian men who attend these support groups. PATIENTS AND METHODS In all, 1224 men (51% response) from 44 prostate-cancer support groups across Australia were recruited by mail. Men completed self-report measures that included the Prostate Cancer Peer Support Inventory (PCSI), the UCLA Prostate Cancer Index bother scales, psychological distress, quality of life (QoL), bother from pain and tiredness, perception of the clinician's support for group participation. Group-level variables were also included in the analyses. RESULTS Peer support was rated positively by most men; a high satisfaction with support groups was related to better QoL, lower pain, younger age, higher perceived clinician support for group participation, use of alternative therapies, lower education, and regular attendance; dissatisfaction with support groups was related to higher psychological distress, lower QoL, and lower perceived clinician support for group participation. Group variables did not predict positive or negative support. Overall QoL was similar to community norms and psychological distress was low, with only 8% of men reporting high distress. The most common physical symptom was sexual bother, with 74% of men reporting moderate or high bother. CONCLUSIONS The PCSI was a useful measure of peer support. Perception of the benefits of peer support was related to individual but not group differences. The clinicians' attitudes to participation in support groups influenced the men's experience of these groups, and this finding has implications for developing support services for these men.