High-achieving (HA) and low-achieving (LA), learning disabled (LD), and attention deficit hyperactivity disorder (ADHD) high school students were followed over two years of L2 study and compared on measures of L1 literacy (reading and writing) in elementary school, L1 cognitive ability, L2 aptitude, oral and written L2 proficiency, and L2 word decoding and spelling, and in L2 grades. Findings revealed significant differences between the HA and LA and the HA and LD groups on all of the L1 and L2 measures. No significant differences were found between the LA and LD groups on any of the L1 and L2 measures, and few differences were found between the HA and ADHD groups. Results replicated studies which have shown that L1 literacy skills are important for L2 learning. New findings showed that students classified as ADHD who have average or better L1 skills do well in L2 courses and that L1 literacy differences between high and low achieving L2 learners emerged early in elementary school, providing support for long-term cross linguistic transfer of L1 to L2 skills.