To determine whether reading and writing are as closely related as commonly supposed, a study estimated the amount of overlap that exists between several components of reading and writing knowledge. Data were obtained from 256 second graders and 251 fifth graders. Reading measures included tests of phonics (word knowledge), vocabulary (lexical knowledge), sentence comprehension (syntactic knowledge), and passage comprehension (organizational or structural knowledge). Writing measures included assessments of spelling ability (word knowledge), vocabulary diversity (lexical knowledge), sentence structure complexity (syntactic knowledge), and story grammar structure (organizational or structural knowledge). In all, there were eight writing measures and four reading measures. Each measure was used as a dependent variable in a separate multiple regression analysis. Results did not support the idea that reading and writing are identical in terms of underlying knowledge. The correlations between the reading and writing variables were significant, but they were much lower than would be expected given the assumptions regarding the relationship between reading and writing. (HOD)