Households that fail to refinance their mortgage when interest rates decline can lose out on substantial savings. Based on a large random sample of outstanding U.S. mortgages in December of 2010, we estimate that approximately 20% of households for whom refinancing would be optimal and who appeared unconstrained to do so, had not taken advantage of the lower rates. We estimate the present-discounted cost to the median household who fails to refinance to be approximately $11,500, making this a particularly large consumer financial mistake. To shed light on possible mechanisms and corroborate our main findings, we also provide results from a mail campaign targeted at a sample of homeowners that could benefit from refinancing.