This study explored the motivation for using "suicide message boards", a type of online bulletin board system (BBS). It classified users according to their motivation, and identified the influence of the suicide BBS on the users' suicidal thoughts. The participants were 137 users of suicide BBSs in Japan, who were asked to answer an online questionnaire about their motivation for accessing and using the BBS. Factor analysis showed that the users' motivations included four factors: helping, unburdening oneself, finding a way out, and preparing for suicide. Cluster analysis showed that users were divided into four groups. Cluster 1 had a high level of motivation for helping others. Cluster 2 had no specific motivation. Cluster 3 was motivated to counsel each other. Cluster 4 had the highest suicidal motivation. Furthermore, in all groups suicidal thoughts were decreased by BBS usage, and the reduction of suicidal thoughts in groups with a strong motivation to help other users was more than in other groups.