Background: Learning Progress Assessments (LPA) have been developed to help teachers individualize their curriculum. The use of LPA is facilitated by an increasing number of computerized LPA tools. However, little is known about student factors that influence the effectiveness of computerized LPA. Objectives: In this study, we explored whether a computerized LPA that focused on reading comprehension was differentially effective depending on students' initial reading comprehension abilities. Moreover, effects of the LPA implementation on underlying or related form-based literacy skills (i.e., decoding, spelling) were explored. Methods: The development of reading and spelling skills of 668 third graders was assessed in 41 LPA and 36 control classes in a pretest-posttest design. We used multi-level modelling to analyse effects of the LPA on reading comprehension, decoding, and spelling skills, and tested whether these effects were qualified by students' initial achievement level. Results: The LPA treatment proved beneficial for improving reading comprehension but not for improving decoding or spelling. Children with low levels of reading comprehension at the beginning of the school year benefitted particularly from LPA. Takeaways: Teachers seem to make use of the data offered by the computerized LPA to identify children with reading comprehension difficulties. For these children, an accelerated increase in reading comprehension ability was observed. Results also suggest that this effect is specific to the literacy skill measured by the LPA and does not generalize to underlying or related literacy skills (i.e., decoding, spelling).