Early efforts to apply knowledge of dialect differences to reading stressed the importance of the distinction between differences in pronunciation and mistakes in reading. This study develops a method of estimating the probability that a given oral reading that deviates from the text is a true reading error by observing the semantic impact of the given pronunciation on the child's reading of the text that immediately follows. A diagnostic oral reading test was administered to 627 children who scored in the 33rd percentile range and below on state-mandated assessments in reading in Philadelphia, Pennsylvania, Atlanta, Georgia, and California elementary schools. Subjects were African American, European American, and Latino, including Latinos who learned to read in Spanish and in English first. For 12 types of dialect-related deviations from the text that were studied, the error rates in reading the following text were calculated for correct readings, incorrect readings, and potential errors. For African Americans, many of these potential errors behaved like correct readings. The opposite pattern was found for Latinos who learned to read in Spanish first: most types of potential errors showed the high percentage of following errors that is characteristic of true errors.