The Slosson-Diagnostic Math Screener (S-DMS) was designed to help identify students in Grades 1 to 8 at risk for mathematics failure. Internal consistency, test-retest reliability, item analysis, decision efficiency, convergent validity, and factorial validity of all five levels of the S-DMS were studied using 20 independent samples of students aged 6 to 13 years. The Woodcock-Johnson Tests of Achievement-Revised Broad Mathematics Cluster, the Wide Range Achievement Test-Revised Arithmetic subtest, and the KeyMath-Revised Operations Area were used as convergent validity measures. (Contains 7 tables and 1 figure.)