Selection and socialization have been implicated in friendship homophily, but the relative contributions of each are difficult to measure simultaneously because of the nonindependent nature of the data. To address this problem, the authors applied a multiple-groups longitudinal actor-partner interdependence model (D. A. Kashy & D. A. Kenny, 2000) for distinguishable dyads to 3 consecutive years of intoxication frequency data from a large community-based sample of Swedish youth. Participants, ranging from 12 to 18 years old (M = 14.35, SD = 1.56) at the start of the study, included 902 adolescents (426 girls and 476 boys) with at least one reciprocated friend during at least one time point and 212 adolescents (84 girls and 128 boys) without reciprocated friends at any time. Similarity estimates indicated strong effects for selection and socialization in friends' intoxication frequency. Over time, younger members of these dyads had less stable patterns of intoxication than older members, largely because younger partners changed their drinking behavior to resemble that of older partners. (Contains 10 figures, 1 footnote, and 2 tables.)