We examined commonalities and differences of close relationships among the Americans and Japanese by using three major propositions extracted from the individualism/collectivism concept. Responses to three kinds of social relationship scales, i.e., affective, instrumental, and conflict, were compared. Americans (N = 547) and Japanese (N = 808) of 20 to 64 years of age rated at least four significant others out of five, i.e., mother, father, partner/spouse, a child, and same-gender friend. Findings obtained by three kinds of comparisons-(1) mean scores; (2) correlation patterns; and (3) the dominant figures-indicated both commonalities and differences across the cultures. Whereas nearly half of the working hypotheses based on the three propositions derived from the individualism/collectivism concept were supported by the data, others revealed differences in the opposite direction. It is discussed that the individualism/collectivism framework may be a useful tool for cross-cultural comparisons, but not a theory or model of the Western and Eastern cultures.