Fifty-four students were followed over 10 years beginning in first grade to determine best predictors of oral and written second language (L2) proficiency. Predictor variables included measures of first language (L1) skill administered in first through fifth grades, L1 academic aptitude, L2 aptitude (Modern Language Aptitude Test), and L2 affect (motivation, anxiety). All participants completed 2 years of L2 study in high school. Findings revealed strong correlations between early L1 skills and later L2 proficiency, but the Modern Language Aptitude Test was the best predictor of overall L2 proficiency and most L2 proficiency subtests. However, L1 word decoding was the best predictor of L2 word decoding skills. Early L1 skills, L2 motivation, or L2 anxiety added a small amount of variance to the prediction models. Findings suggested that language-related variables are the most robust predictors of L2 proficiency. Results are discussed in the context of long-term cross linguistic transfer of early L1 skills to later L2 aptitude and L2 proficiency.