A study was conducted to examine the effects of prior knowledge on the reading comprehension performance of students with learning disabilities. Instruction in information and vocabulary concepts was provided to 13 junior-high school students with learning disabilities who had been predetermined to lack the prior knowledge required by an experimental test of reading comprehension. The effect of prior knowledge was examined by comparing the performance of an experimental group (high prior knowledge group) to a control group (low prior knowledge group). The effect of text structure was also examined by comparing reading comprehension performance on three types of reading passages - textually explicit, textually implicit, and scriptually implicit. The results indicated that students in the experimental group increased their prior knowledge and, as a result of instruction, demonstrated superior reading comprehension performance. In addition, text structure was found to affect reading comprehension performance. When comprehension questions tapped information provided in the text, reading comprehension performance improved for all students. Educational implications of the results are discussed.