The author examined whether the level of self-disclosure would differ across four types of relationships--passionate love relationships, companionate love relationships, same-sex friendships, and cross-sex friendships--and across cultures: American culture and Japanese culture. Participants were 145 college students (64 Americans and 81 Japanese). The results supported three hypotheses: (a) Japanese students scored lower in self-disclosure than American students, regardless of relationship types, (b) self-disclosure was higher in same-sex friendships than in cross-sex friendships both among American participants and among Japanese participants, and (c) self-disclosure was higher in romantic relationships than in friendships both among American students and among Japanese students. However, the correlation between self-disclosure and passionate love was not stronger than the correlation between self-disclosure and companionate love. The author discussed the present study's findings and contribution.