PURPOSE: The aim of the present randomized intervention study was to investigate the effect of receiving extended cancer genetic information on counselees' knowledge, risk perception, information sharing and satisfaction with the service. METHODS: In total, 147 counselees, affected by cancer and/or a family history of cancer, were randomized to extended or standard information. The levels of counselees' knowledge and personal risk estimations were measured at four time points. In addition, counselees' satisfaction with the counseling and sharing of the information to at-risk relatives was assessed. The intervention included meeting a specialist nurse, learning the breaking bad news method, receiving written material and video-taped counseling sessions. RESULTS: A significant increase in the level of knowledge in participants in the "breast cancer group" regardless of the randomization was observed over time. The correct estimation of personal risk increased significantly in both groups after two weeks, but declined at the eight month follow-up. Most of the participants had informed at-risk relatives about their visit at the cancer genetic clinic. The majority of respondents in both groups were highly satisfied with the counseling. The only observed effects of the intervention were that counselees in the intervention group were significantly more satisfied with the content of the given information and with the way of informing relatives. CONCLUSION: Apparently, the current genetic counseling is managed properly and extended information does not seem necessary in all cases. However, some counselees need additional sessions.