The present study investigated the effects of varying lengths of overseas experiences on 37 Japanese students' English writing ability and motivation over 3.5 years. The students were observed at the beginning of their first year and in the middle of their second, third, and fourth years at their university. During the 3.5-year observation period, 28 of the 37 students spent 1.5 to 11 months in English-speaking countries. The results revealed that (1) students' second language (L2) writing ability did not change in a linear way; (2) over the 3.5 years, students who spent some time abroad significantly improved their L2 writing ability whereas those who stayed in Japan did not; (3) many of those students who went abroad formed L2-related "imagined communities" that possibly motivated them to improve their L2 writing ability; (4) those students who spent more than 4 months abroad improved their L2 writing ability significantly more than the other students; and (5) only those students who spent more than 8 months abroad became intrinsically motivated and voluntarily practiced to improve their L2 writing. (Contains 6 footnotes and 3 tables.)