We investigated developmental trajectories of and the relation between reading and writing (word reading, reading comprehension, spelling, and written composition), using longitudinal data from students in Grades 3-6 in the US. Results revealed that word reading and spelling were best described as having linear growth trajectories whereas reading comprehension and written composition showed nonlinear growth trajectories with a quadratic function during the examined developmental period. Word reading and spelling were consistently strongly related (0.73 = rs = 0.80) whereas reading comprehension and written composition were weakly related (0.21 = rs = 0.37). Initial status and linear slope were negatively and moderately related for word reading (- 0.44) whereas they were strongly and positively related for spelling (0.73). Initial status of word reading predicted initial status and growth rate of spelling; and growth rate of word reading predicted growth rate of spelling. In contrast, spelling did not predict word reading. When it comes to reading comprehension and writing, initial status of reading comprehension predicted initial status (0.69), but not linear growth rate, of written comprehension. These results indicate that reading-writing relations are stronger at the lexical level than at the discourse level and may be a unidirectional one from reading to writing at least between Grades 3 and 6. Results are discussed in light of the interactive dynamic literacy model of reading-writing relations, and component skills of reading and writing development.