Underperformance in mathematics has been an issue that plagues the education system in Jamaica. Studies in first world countries have shown that enrichment programs, including Mathematics Enhancement Programmes (MEPs,) have been positively impacting attainment in mathematics. This quasi-experimental research design study investigated the impact of an MEP on Jamaican students' attainment in mathematics. A sample of seven grade one classes from two primary schools in representative areas in Jamaica were selected for the intervention group. The treatment involved teaching the Jamaican grade one mathematics standards using the MEP resources for nine months. A statistically significant improvement and large effect size of the intervention was found, indicating that the MEP had a substantial impact on students' achievement and attitudes towards mathematics. This study has implications for designing enrichment programs geared at addressing mathematics underperformance in Jamaica and in similar countries.