This study describes a benefit-cost analysis of a comprehensive mentoring program for beginning teachers conducted in a medium-sized California school district. Using actual program cost information and data on student achievement, teacher retention, and mentor evaluations, the authors performed a benefit-cost analysis to determine whether comprehensive mentoring for beginning teachers makes financial sense. The data showed that, contrary to expectations, increases in teacher effectiveness yielded greater savings than the reduction in costs associated with teacher attrition. Overall, the benefit-cost analysis showed that, after 5 years, an investment of $1 produces a positive return to society, the school district, the teachers, and the students, and the state almost recovers its expenses. Implications are drawn for both education and public policy. (Contains 7 tables, 2 figures and 2 footnotes.)