Purpose The purpose of this study was to investigate the relative effectiveness of an individualized laboratory approach upon the mathematics achievement of underachieving third-grade students performing below grade level. In the study, achievement of students receiving mathematics instruction in an individualized laboratory was compared with achievement of students receiving traditional mathematics instruction in the classroom. A similar comparison of the mathematics achievement of male and female subjects was also included in the investigation. Procedures Mathematics laboratories were established in two elementary schools designated as ESEA, Title I schools in terms of socio-economic deprivation. From these two schools, an experimental group was selected consisting of 62 third-grade students achieving in the lower three stanines as measured by pretest scores on the Metropolitan Achievement Test. A control group was formed by selecting 62 subjects from three other Title I schools to match the experimental group in terms of sex, race, and achievement in mathematics. The treatment period for the experimental group consisted of 45 minute daily sessions in the laboratory for a period of seven weeks. The laboratory program consisted of a systematic method of diagnosis, prescription, evaluation, and recording of individual progress with utilization of a wide variety of materials and equipment to match the learning styles and rates of laboratory participants. The control group received traditional mathematics instruction daily in the classroom for the same treatment period. The mathematics section of the Metropolitan Achievement Test, including subtests on computation skills and concept development and problem-solving skills, and the Criterion Referenced Tests were administered as pretest and posttest measures to both groups. A three-factor analysis of variance procedure with repeated measures on one factor was used to analyze data obtained. F tests were used, whenever appropriate, to determine the significance of the pretest-posttest differences for both treatment and sex. A significance level of .05 was selected as the basis for acceptance or rejection of the hypotheses in the study. Findings Findings of this study indicated that students receiving instruction in an individualized mathematics laboratory demonstrated significant gains (α= .01) in: (1) total mathematics achievement and concept development and problem-solving skills as measured by the mathematics section of the Metropolitan Achievement Test, and (2) mastery of specific skills as measured by the Criterion Referenced Tests. No significant difference was found between the two groups in regard to scores on the computation skills subtest of the Metropolitan Achievement Test. In addition, no significant differences in achievement were found for males as compared to females on either of the tests. Conclusions With the exception of improvement in computation skills, findings of this study indicated that the individualized laboratory approach significantly improved the mathematics achievement of underachieving third-grade students. However, the individualized laboratory approach did not significantly affect achievement of males as compared to females. Implications Further research is recommended; (1) utilizing a broader sample and longer treatment period, and (2) involving different grade levels and socio-economic groups. / Ed. D.