Abstract: Americans now hold trillions of dollars in individual retirement savings accounts. Concerned about conflicts of interest among financial advisers who provide advice to retirement savers, the Department of Labor has proposed imposing fiduciary status and a "best interest" standard on such advisers. To ameliorate the resulting compliance costs, the DOL has also raised the possibility of a safe harbor for certain "high-quality low-fee investments." However, the notion of a "high-quality" investment is in irreconcilable tension with the highly individualized assessment of risk and return that is required by modern portfolio theory, the well-accepted concept from financial economics that has been codified in the "prudent investor rule" as the standard of care for fiduciary investment. This policy incoherence is worrisome because of the potential for the safe harbor to swallow the best interest standard.