This study examines the problem of underachievement among gifted high school students. Low achievers were compared to high and moderate achievers on their motivation, self-regulation, and attitudes toward their school and teachers. Participants were all highly able students from grades 10 and 11 in an academically selective gifted high school in Australia (n=197). Teachers were asked to rank the students into high, moderate, and low achievers in terms of their performance in two subjects English and Mathematics. Participants were asked to respond to two surveys that measured their personality characteristics. The results indicate that math achievement and not language achievement may be used with confidence to classify gifted students; high achiever had higher mean scores than moderate and low achievers on all study variables; intrinsic motivation then extrinsic motivation had the highest correlation with math achievement and can be used to differentiate males and females performance.