Adolescent drinking has an important health and social impact in many countries. In Spain, this behavior often takes place in groups and in open areas (known as "botellón"). The aim of this study is to describe the prevalence of excessive drinking among Spanish adolescents and its association with socialization and family factors. A national school survey was conducted in 2006 among 26,454 students aged 14-18 years who were selected by two-stage cluster sampling (schools and classrooms). The questionnaire was self-completed with paper and pencil. The outcomes were: habitual excessive drinking or HED (average consumption ≥30 g/day of alcohol among men, and ≥20 g/day among women), binge drinking (drinking 5 or more standard alcohol units in a 2-hour interval), and drunkenness. Logistic regression models were used to estimate the effect of socialization and family factors. Monthly prevalence of HED, binge drinking and drunkenness was 11.2%, 30.9% and 25.6%, respectively. The main factors positively associated with HED were: frequently going out for fun in the evenings, high proportion of friends who drink or get drunk, early onset of alcohol use, low perceived risk of drinking, truancy, illegal drug use, and amount of money spent for personal needs. Family factors were weakly associated with outcomes. Socialization in leisure environments with friends who drink excessively is an important predictor of adolescent excessive drinking in Spain. Thus, prevention must also focus on the community level, limiting alcohol access, building socialization environments without alcohol, and increasing adolescents' risk perception of drinking.