This study examined the effects on math performance of the use of the rekenrek, a manipulative developed by Adrian Treffers. The rekenrek looks like an abacus, but differs in that it is based on a five-structure and not a ten-structure system. It is comprised of two rows of 10 beads, each broken into two sets of 5 by color (i.e., in each row the first 5 beads are red and the next 5 are white). Forty-five students with learning disabilities were divided into three equivalent groups based on their pretest scores: Group 1 received instruction using rekenrek, Group 2 received instruction without rekenrek, and Group 3 received no instruction. Data were analyzed using one-way ANOVA followed by the Scheffe post-hoc test. Results indicated that students in Group 1 scored significantly higher on an addition and subtraction test with numbers from zero to 20 than did students in either Group 2 or 3. Use of multiple-regression analysis confirmed that the results were significant and meaningful. The advantages of the rekenrek and the implications of the study for math instruction within the Realistic Mathematics Education model are addressed. (Contains 4 figures and 4 tables.)