Psychological studies of loneliness are comparatively new. This study examined influences of attachment style, family functions and gender differences on loneliness in Japanese university students. The following questionnaires were administered: University of California at Los Angeles (UCLA) Loneliness Scale, Internal Working Model Scale for assessing attachment, and Family Adaptability and Cohesion Evaluation Scale III. Stepwise multiple regression analyses were employed to examine relationships between loneliness as the dependent variable, and gender, attachment style and family functions as independent variables. Results showed that gender differences significantly influenced loneliness. Furthermore, loneliness was positively influenced by avoidant and ambivalent attachment styles and negatively influenced by stable attachment style and family cohesion. A significant interaction was also observed between secure attachment style and family cohesion. These results indicated that a stable attachment style might reduce loneliness. On the other hand, in spite of an unstable attachment style, loneliness can be decreased if family cohesion is high.