In a sample of 1,897 youth studied across the last year of elementary school to the second year of high school, we identified five trajectories of drinking frequency. Three of those (nondrinkers, middle onset, and late onset drinkers) were not drinking in elementary school; two others (moderate drinkers and early high drinkers) were. Among originally nondrinking groups, multiple impulsigenic traits and the acquired preparedness risk model predicted membership in groups that subsequently began drinking. Membership in trajectory groups characterized by drinking during this age period was associated with (a) the experience of alcohol-related problems and (b) further increases in both impulsigenic traits and alcohol expectancies. Youth vary considerably in the development of drinking behavior across the transitions from elementary to high school. Harms associated with early drinking involve both problems from drinking and increases in high-risk personality traits.