The objective of this study was to investigate the contribution of rapid digit naming, phonological memory, letter sound naming, and orthographic knowledge to the prediction of responsiveness to a school-based, individual intervention of word reading fluency problems of 122 Dutch second and third graders whose reading scores were below the 10th percentile in comparison with the normative group. Degree of responsiveness was determined by comparison of a pre- and posttest measure of word reading fluency with a 6-month interval. At posttest, 38% of the children had improved their reading scores above the 10th percentile. Maintenance scores revealed no significant growth on average, confirming that word reading fluency skills of poor readers are hard to remediate. Except rapid digit naming, none of the measures predicted responsiveness after controlling for the autoregressive effect of initial performance on fluency of word reading. A large part of the variance remained unexplained, supporting the advantage of a response-to-intervention approach above traditional psychometric testing to identify severe reading disabilities.