The purpose of this study was to examine how the family of origin and self-disclosure of unmarried persons in early adulthood affected their own intimacy with the opposite sex, which was one of the major developmental tasks for them. The findings of the study are as follows: First, the self-differentiation, self-disclosure, and intimacy with the opposite sex of the single male and female adults investigated were generally at a moderate level. Second, for self-differentiation, the background variables of the male and female adults made no statistically significant difference. For self-disclosure, the background variables of the female adults made no statistically significant difference to that, but the male adults differed significantly from one another in that regard according to their family makeup and birth of order. As for connections between the background variables of the single male and female adults and their intimacy with the opposite sex, the male adults varied in that aspect with their birth order, and whether they were dating and considered getting married made a difference to that as well. In the event of the female adults, whether they considered getting married and how long they were dating made a difference in their intimacy with the opposite sex. Third, concerning the relative influence of the self-differentiation and self-disclosure of the unmarried male and female adults on their intimacy with the opposite sex, their intimacy with the opposite sex improved as division occurred from their original families, when they opened themselves up to more people, when they thought of marriage more and when they were dating for a longer period of time.