Purpose: Using a Problem Behavior Theory framework, this article examines the extent to which psychosocial correlates of early sexual initiation (before age 16) vary across developed nations.Methods: Fifteen-year-old participants (n = 5,624) in the 1997-1998 World Health Organization collaborative Health Behavior in School-Aged Children survey (Finland, Scotland, France, and Poland) and the 1996 U.S. Add Health survey self-reported substance use (alcohol and tobacco), school attachment, positive parental communication, and early sexual intercourse experience. Stratifying by gender, we performed univariate, bivariate, and multivariable analyses controlling for family socioeconomic status, family structure, and nation fixed effects.Results: Self-reported early sexual experience, substance use, school attachment, and positive communication with parents varied significantly across nations for both boys and girls. In both crude and adjusted analyses, substance use was positively associated with early sexual experience among boys and girls across nations, although associations were stronger in Europe than in the United States (adjusted odds ratio [AOR]; range, 1.56-3.74). School attachment was similarly inversely related to early sexual experience among boys and girls across nations (AOR range, .63.94). However, positive parent communication was significantly inversely related to early sexual experience only among U.S. females (AOR .50).Conclusions: Findings overall supported the fit of early adolescent sexual initiation as a risk behavior within a Problem Behavior Theory framework cross-nationally, suggesting that similar factors could be targeted to prevent early sexual initiation across some developed nations. However, further research is warranted examining the temporality of these relationships. (C) 2010 Society for Adolescent Health and Medicine. All rights reserved.