Using an instrument for U.S. Treasury supply, we show that convenience yields on medium- and long-term Treasurys are much more sensitive to supply than those on short-term Treasurys, which respond little. Over the past two decades, fiscal expansion has substantially reduced medium- and long-term convenience yields, at times pushing them below zero, while short-term convenience yields have declined toward zero. Quantitatively, we estimate that increased Treasury supply explains a large share of the secular decline in medium- and long-term convenience yields, lowering the total seigniorage revenue from Treasury issuance by about one-third.