Research on the effects of Study Abroad (SA) periods on learners’ linguistic progress has tended to focus on oral skills, and few SA studies have focused on learners’ development in writing while abroad. The subjects in the present study were 37 advanced level non-native (NNSs) university students of English on a SA programme. Written compositions were analysed for fluency accuracy and complexity gains after the SA. They were contrasted with progress after formal instruction and with native speakers (NSs) baseline written performance. Language background data, attitudes, and stay abroad conditions were elicited with questionnaires. The SA period resulted in significant progress, which receded in the mid-term. Formal instruction only showed some improvement in accuracy. Significant differences were found between NSs and NNSs, although not in all domains. Results allow us to identify the students who benefit most from the SA and to examine the factors which seem to characterize them.