Researchers and policymakers are increasingly interested in the impact of afterschool programs on youth development. Even though numerous studies have investigated the impact of afterschool participation on academic outcomes, there is limited research on the differential impact of afterschool programs based on students' participation rate. This study bridges that research gap and presents results from a study of the effectiveness of the LA's BEST (Los Angeles's Better Educated Students for Tomorrow) afterschool program based on different levels of student participation. This research tracked 4 years of the academic histories for two cohorts of students participating in LA's BEST. We separated the students in each cohort into four categories based on their intensity of attendance in LA's BEST and then used a propensity based weighting method to remove existing differences in student background characteristics. Hierarchical growth modeling was employed to analyze the academic outcomes. Results indicate that math achievement outcomes of students vary by intensity of program participation. Student participants who attended LA's BEST over 100 days per year demonstrated greater math achievement growth than students with low program attendance. This finding was consistent, and was statistically significant, for both cohorts of students. In contrast, although the trend for English-language arts achievement growth was positive, and followed a developmental pattern similar to math, it did not vary significantly by intensity of program participation. This finding was also consistent for both cohorts of students. A technical appendix is included. (Contains 16 tables, 4 figures and 15 footnotes.) [This work was supported by grant number 021891 from LA's BEST with funding to the National Center for Research on Evaluation, Standards, and Student Testing (CRESST).]