Previous studies have used cross-sectional designs to demonstrate the beneficial effect of acquaintanceship on the validity of personality impressions. To counter critiques of those studies, a longitudinal design was used. Participants were randomly assigned to 16 groups of 5-7 members who met once a week for 7 weeks. None of the participants in any group were previously acquainted. Before the first meeting, they completed a battery of self-report measures, including the NEO Five Factor Inventory and the revised Interpersonal Adjective Scales. After Weeks 1, 4, and 7, group members rated each other on single-item measures related to each of the Big Five. All correlations between self-reports and corresponding peer ratings (i.e., validities) were significant by Week 7. The mean Big Five validity increased significantly from.21 to.26 to.30 at Weeks 1, 4, and 7, respectively Extraversion showed the highest validity and consensus.