This case study investigates the impact of cognitive style (field in/dependence) on errors in English writing through extensive reading (ER). It explores different possibilities between two field-independent (FI) and field-dependent (FD) learners through ER, and it considers the types of errors (interlanguage or intralingual) that they are more likely to commit. The participants were two Korean female high school students with different cognitive styles. To ascertain their respective cognitive styles, the Group Embedded Figures Test (GEFT) was conducted. The participants were asked to write a reading log once a week for six months. Data analyses were performed by two raters, and for further qualitative analysis, Nvivo 10 was used. The results showed that ER was more beneficial to the FI learner with regard to writing proficiency, and both the FI and FD learners committed considerable interlanguage errors. Furthermore, there were similarities and discrepancies between the FI and FD learners as a result of the ER treatment in interlanguage error change.