This study was designed to examine associations between family relationship quality and friendship quality in the United States and Japan using surveys of representative samples of individuals ranging in age from 13 to 93 in the United States (n = 1498) and Japan (n = 1641). Associations between family relationships and friendships were stronger and more pervasive in Japan than in the United States. In the United States, there was some evidence that having higher quality family relationships was associated with having higher quality friendships; in Japan, there was similar evidence for consistency in quality across relationships, as well as evidence that qualities lacking in one relationship were sometimes present in another. Gender and age did not moderate the associations between family relationship and friendship quality in either country. These findings extend into adulthood work that has previously focused on family relationships and friendships of children and suggest cultural differences in patterns of association between family relationships and friendships.