Our objective is to understand how out-of-pocket costs for contraceptives affect take-up among low-income, uninsured women. We randomized women seeking care at Planned Parenthood to receive vouchers for the purchase of any contraceptive method. The intervention group received either a voucher valued at 50% or 100% of the out-of-pocket costs for an IUD. The control group received no voucher and received usual clinical care. Relative to the control group, 100%-voucher recipients were 40% more likely to buy contraceptives; spent 63% more on contraceptives; took up long-acting, reversible contraception (LARC) 310% more often; had 0.185 fewer expected annual method failures; and increased contraceptive coverage by 325 days. Findings hold for all pre-specified subgroups except Black women, whose take-up of LARCs was significantly lower. Results suggest that making contraceptives free for all Title X patients could reduce unintended pregnancies by 12% and births by 4%, annually.