Abstract Study question Is home-based monitoring of ovulation cost effective compared with hospital-controlled ovulation in women undergoing frozen embryo transfer in the natural cycle? Summary answer Home-based monitoring to time frozen-embryo-transfer is cost-effective compared to hospital-controlled ovulation, as it results in lower cost with no difference in ongoing pregnancy rates. What is known already Women undergoing frozen embryo transfer treated by home-based monitoring of ovulation or hospital-controlled ovulation have comparable ongoing pregnancy rates. We found no differences in secondary outcomes, including the risk of undetected ovulation. Complete home-based monitoring of ovulation with LH hormone kits is feasible and requires no hospital visits at lower costs since ultrasound monitoring and hCG injection are no longer needed. Both consequences imply potentially lower costs after home-based monitoring of ovulation. Study design, size, duration We performed an economic evaluation of home-based monitoring of ovulation compared with hospital-controlled ovulation in a national multicentre randomised controlled trial in women undergoing frozen embryo transfer (ANTARCTICA-2 RCT Dutch TrialRegister Trial NL6414). 1464 women were randomly assigned between April 10, 2018, and April 13, 2022, with 732 allocated to home-based monitoring and 732 to hospital-controlled monitoring. All randomized patients were included according to the intention-to-treat principle. Participants/materials, setting, methods We performed a cost-effectiveness analysis from a hospital and societal perspective. We compared the direct medical costs of home-based monitoring of ovulation and hospital-controlled ovulation during one cycle until an ongoing pregnancy occurred. Direct medical costs included digital ovulation tests, transvaginal ultrasound monitoring of the dominant follicle followed by hCG trigger, hormone measurements in blood, treatment for miscarriage or ectopic pregnancy. Nonparametric bootstrapping was used to calculate cost estimates and differences with 95% confidence intervals. Main results and the role of chance Ongoing pregnancy rates were 20·8% in the home-based monitoring group and 20·9% in the hospital-controlled group (risk ratio [RR] 0·99 [90% CI 0·81 to 1·22]; risk difference [RD] -0·14 [90% CI -3·63 to 3·36]). No differences in ectopic pregnancies or miscarriages were found. Home-based monitoring required less travelling to the hospital, less ultrasounds by doctors or nurses, less hormone measurements in blood, and less medication (hCG injections). Mean direct medical costs per woman in the home-based monitoring of ovulation group and in the hospital-controlled ovulation group were €752 versus €991, respectively, with a mean difference of €239. The ICER consistently showed home-based monitoring to be less costly with no difference in effectiveness. Consequently, the cost difference from a societal perspective was larger. The mean costs per woman in the home-based monitoring of ovulation group and in the hospital-controlled ovulation group were €787 versus €1395, respectively, with a mean difference of €607 in favour of home-based monitoring. Limitations, reasons for caution We report on one cycle per women. Women usually undergomultiple frozen-embryo-cycles. This is of importance for clinical decision making aboutimplementation of home-based monitoring, since the cumulative benefit of multiple cycles perwoman should be taken into account. Wider implications of the findings The results of our trial show that home-based monitoring time frozen embryo transfer is equally effective but less costly compared with hospital-controlled ovulation. In this era of climate change and shortages of hospital staff and waiting lists, any process that takes less human labour is important. Trial registration number Dutch Trial Register (Trial NL6414)