The objective of the study was to evaluate whether mental health problems identified through screens administered in first grade are related to poorer academic achievement test scores in the fourth grade. The government of Chile uses brief teacher- and parent-completed measures [Teacher Observation of Classroom Adaptation-Revised (TOCA-RR) and Pediatric Symptom Checklist (PSC-Cl)] to screen for mental health problems in about one-fifth of the country's elementary schools. In fourth grade, students take the national achievement tests (SIMCE) of language, mathematics and science. This study examined whether mental health problems identified through either or both screens predicted achievement test scores after controlling for student and family risk factors. A total of 17,252 students had complete first grade teacher forms and these were matched with fourth grade SIMCE data for 11,185 students, 7,903 of whom also had complete parent form data from the first grade. Students at risk on either the TOCA-RR or the PSC-Cl or both performed significantly worse on all SIMCE subtests. Even after controlling for covariates and adjusting for missing data, students with mental health problems on one screen in first grade had fourth grade achievement scores that were 14-18 points (~1/3 SD) lower than students screened as not at risk. Students at risk on both screens had scores that were on average 33 points lower than students at risk on either screen. Mental health problems in first grade were one of the strongest predictors of lower achievement test scores 3 years later, supporting the premise that for children mental health matters in the real world.