Findings show that the population incidence rates of marijuana use increased from 1.30 percent to 16.29 percent from age 11 to 16 and then apparently stabilize. A sharp increase occurred between ages 13-15. Among six identified important factors, alcohol and/or tobacco use and marijuana offers were the most important risk factors across ages and age cohorts. Consistent with hypotheses, parental influence and peer influence varied as youth aged. Both parental influence and peer influence had significant effects during early adolescence, and peer influence continued up to middle adolescence. Parental monitoring was a protective factor in countering peer influence on marijuana initiation, but parental influence had little effect during late adolescence. These findings thus provide some empirical evidence for a shift from parental influence to peer influence. The study obtained longitudinal data from 7 nationally representative age cohorts (ages 10-16 years old) of youth who had never used marijuana (n=4,607) and their parents. These data were collected during 1999-2004, using the National Survey of Parents and Youth. The survey was designed to measure changes in drug-related beliefs, attitudes, and behaviors in American youth and their parents. A series of lagged logistic regression analyses were performed with a cohort-sequential design. Multiple imputation was used to handle missing data, and longitudinal replicate weights were incorporated into the analyses. 4 tables, 3 figures, a glossary, and 25 references