The matter of creating topics that will elicit writers' optimal performance is fundamental to writing assessment. However, topic variables (i.e., wording, mode of discourse, rhetorical specification, and subject matter) have only recently begun to be investigated, and a clear understanding of the relationship to these variables to writing performance has yet to be achieved. One topic variable-subject matter-was the focus of the present study. The extent to which ESL graduate students' writing performance was affected by their knowledge of the subject matter of the assessment topic was investigated. A total of 105 students representing three ESL course levels participated in the study. All subjects responded to two topics-one general, and one pertaining to the subjects' fields of study. The essays were scored on the basis of holistic measures, length, and T-unit and error-free T-unit indices. The highly significant results obtained in the statistical analyses indicated that, in general, writing performance on the field-specific topic was superior. Similarly, the field-specific topic was found to be superior to the general topic in terms of its ability to discriminate among groups having different levels of writing proficiency.