This study explored the background knowledge (BK) and reading comprehension (RC) relationship for struggling adult readers. Using confirmatory factor analyses, a single-factor BK model exhibited better fit than a two-factor model separating academic knowledge and general information, which indicates that BK represents a unidimensional construct for this population. In addition, one measure of oral vocabulary loaded with a latent factor of BK, which was separable from a latent factor of two additional oral vocabulary subtests. Using structural equation modeling, we found that BK exhibited a direct effect on RC after controlling for participants' grade level, decoding, listening comprehension, and oral vocabulary. [For the corresponding grantee submission, see ED582388.]