This journal article deals with a partly longitudinal study that applies the theoretical framework of Systemic Functional Linguistics to second language writing to investigate the use of grammatical metaphor (defined as non-congruent realizations of meaning, e.g. nominalizations). 365 texts written by Swedish mono- and multilingual students in grades 9 and 11 were examined. The analysis shows that older students and monolingual students make greater use of grammatical metaphor than younger students and multilingual students. Multilingual students with early and late ages of onset use grammatical metaphor more than multilingual students with onset ages between these two extremes. Moreover, a relation was found between the occurrence of grammatical metaphor and the use of the potential functions of grammatical metaphor, e.g. expansion of the nominal phrase, which was used more frequently in texts with a higher density of grammatical metaphor, contributing to the construction of specialized, educational knowledge. Finally, the occurrence of grammatical metaphor was compared to the occurrence of L2 deviations in a subcorpus, and the results interpreted in relation to the Interdependence Hypothesis formulated by Cummins (1979).