This study examined the relationship of interpersonal solidarity to various aspects of idiomatic communication in friends’ relational cultures. A pilot study determined that the typology of idiom functions used in past investigations of romantic relationships could be extended to friendship. The idioms described functioned to name activities, emotional states, objects, and places; communicate affection; manage confrontations; accomplish greetings and goodbyes; reference the self, partner, or other individuals; issue requests and insults; and denote sexual matters. In the main study, 114 females and 117 males each identified a close same-sex friend, completed the Interpersonal Solidarity Scale, and filled out an idiom report form for each idiom that had emerged in the friendship. The majority of the 1,380 idioms reported were verbal, used in public contexts, and used in more than just the one friendship described. For both sexes, solidarity was positively correlated with five idiom categories: activities, affection, confrontation, nicknames for self, and objects. In addition, solidarity correlated positively with place, requests, nicknames for friend, and sexual references for females and with emotions, greetings/goodbyes, labels for others, and teasing insults for males. For both sexes, solidarity was related to the total number and breadth (diversity) of idioms reported, as well as to other features of friends’ idiomatic codes. A subset of the idiom variables collectively accounted for over one third of the variance in, solidarity. Support was also found for several hypothesized sex differences in the structure and use of idiomatic codes. Associations among idiom function, channel, patterns of use, and relational consequences were also explored.