The main research questions of this study were (1) How long have adults in the Netherlands and the United States known members of their nonkin networks? (2) What are the predictors of long-standing nonkin relationships? and (3) Which predictors are recognizable in both societies? The data came from the NESTOR-LSN survey (3,229 adults aged 55 to 89 years in the Netherlands) and from the Northern California Community Study (n = 1,050, with 225 respondents aged 55 to 91 years in the United States). In both countries, the duration of nonkin relationships was related to the absence of network-disturbing variables (e.g., the number of years since the last move), network-sustaining variables (e.g., distance to nonkin), and other network properties (e.g., homogeneity). Nationally based differences were also observed (e.g., having a car was related to stable relationships only in the United States, and the special integrative functions of exclusive friendships were elicited only in Europe).