Abstract: The current study investigates the effects of topical knowledge on English language writing performance of 46 Iranian ESP economics students across different levels of English proficiency (elementary, intermediate, and advanced). The participants were asked to write two essays one on about a topic specific to economics and the other pertaining to a general topic. Their writing performances were scored using three components: language, organisation, and content. The overall analyses showed that all the participants performed significantly better on the specific topic. However, compared to intermediate and elementary participants, advanced learners were found to perform significantly better on the two topics in terms of language, organisation, and content. Also, elementary and intermediate learners performed equally on the specific topic with reference to its content. However, intermediate learners were found to perform significantly better than their elementary counterparts in terms of language and organisation. The interviews also showed that due to its familiarity the specific topic was easy for them to write about. The study brings to attention the importance of topical knowledge or prior familiarity with the content of a test in languages for specific purposes (LSP) which influences the learner's performances to a great extent.