Recent studies of elementary teachers' knowledge about reading have been built on the premise that teachers need thorough knowledge about language and reading processes, but these studies have provided only limited evidence that teachers' performance on tests of such knowledge contributes to their students' reading achievement. The present study was designed to examine the contribution of first- through third-grade teachers' knowledge about early reading to their students' improvement on tests of word analysis and reading comprehension, controlling for socio-demographic characteristics of students, their prior reading achievement, and teachers' educational attainment, professional experiences, and socio-demographic characteristics. Preliminary analyses indicated that the test of teachers' knowledge had adequate psychometric characteristics. However, performance on this measure of teachers' knowledge did not significantly explain students' improvement on the two reading subtests. The complexity of the factors that influence teachers' knowledge acquisition and the context in which the study was carried out offer possible explanations for these results. In addition, teachers' content knowledge about reading might not be closely associated with the practices they use in reading instruction, and therefore might not be significantly related to their students' improvement in reading over a year.