Although several studies have investigated the self-assessment (SA) of writing skills, most research has adopted a cross-sectional research design. Consequently, our knowledge about the longitudinal development of SA is limited. This study investigated whether SA instruction leads to improvement in SA accuracy and in second language (L2) writing. A total of 33 English as a foreign language (EFL) students composed and self-assessed two argumentative essays, one at the beginning (Time 1) and one at the end (Time 2) of a semester-long advanced writing (AW) programme at a Hungarian university. About half of the participants received SA instruction (experimental group), while the other half did not (control group). The essays were scored by two teachers and analysed for linguistic complexity. The results showed improvements in SA accuracy in both groups. However, the SA-teacher assessment (TA) correlation for the total score was statistically significant only in the experimental group at Time 2 (post-instructional phase). Furthermore, the TA total scores and a few linguistic complexity indices showed improvements in L2 writing in both groups. The pedagogical implications of these findings emphasising the importance of SA in EFL writing courses are also discussed.