The aim of this study was to investigate the effect of instructions of curriculum-based measurement (CBM) of reading on (a) the number of words read correctly and incorrectly per minute and (b) the relationship between CBM reading and reading achievement. Results indicated that the specific instructions used have a significant impact on CBM reading outcomes. Statistically significant mean differences were found among the fast, best, and baseline reading conditions in the number of words read correctly and in the number of errors. Correlations between words read correctly per minute and a test of reading achievement were statistically significant and substantial for all three conditions, but differences among their correlations were not. These results underscore the importance of using standardized instructions on CBM results both within and across settings. Implications of these results for the responsiveness-to-intervention method for identifying children with learning difficulties are discussed. (Contains 6 tables and 1 figure.)