Purpose: The purpose of this study was to investigate the relationship between language knowledge at 5 years of age and later academic achievement throughout compulsory school in Iceland. Method: Between 1997 and 1998, 267 Icelandic preschool children aged from 5; 4 (years; months) to 5; 10 were tested with the HLJOM-2 (an Icelandic test of phonological awareness; Simonardottir, Einarsdottir, & Bjornsdottir, 2002) and the Icelandic version of the Test of Language Development-Primary: Second Edition (TOLD-2P; oral comprehension tasks; Simonardottir, Gudmundsson, Skulason, & Petursdottir, 1995). In 2011 these individuals, now aged 18-19 years, were contacted again. Of the original 267 participants, 221 (83%) gave permission to link their results from the preschool language assessments with their performance on national tests in 4th, 7th, and 10th grades. Results: The results showed strong correlation between phonological awareness (as measured by the HLJOM-2) and academic achievement (Icelandic and mathematics) in 4th, 7th, and 10th grades. There was also a significant but lower correlation with oral comprehension skills, as measured with the TOLD-2P. Regression analysis showed that the preschool oral-language assessments in phonological awareness and oral comprehension explained between 35% and 43% of variability in scores on national tests in Icelandic and between 20% and 39% of variability in scores in mathematics. Conclusion: Preschool language knowledge is a reliable predictor of later academic achievement.