BACKGROUND: The new noninvasive finger sensor system NICCI (Getinge; Gothenburg, Sweden) allows continuous cardiac output monitoring. We aimed to investigate its cardiac output measurement performance. OBJECTIVE(S): To investigate the NICCI systemʼs cardiac output measurement performance. DESIGN: Prospective method comparison study. SETTING: University Medical Center Hamburg-Eppendorf, Hamburg, Germany. PATIENTS: Fifty-one patients after cardiac surgery. MAIN OUTCOME MEASURES: We performed a method comparison study in 51 patients after cardiac surgery to compare NICCI cardiac output (CONICCI) and NICCI cardiac output calibrated to pulmonary artery thermodilution cardiac output measurement (CONICCI-CAL) with pulmonary artery thermodilution cardiac output (COPAT). As a secondary analysis we also compared CNAP cardiac output (COCNAP) and externally calibrated CNAP cardiac output (COCNAP-CAL) with COPAT. RESULTS: We analysed 299 cardiac output measurement pairs. The mean of the differences (95% limits of agreement) between CONICCI and COPAT was 0.6 (−1.8 to 3.1) l min with a percentage error of 48%. The mean of the differences between CONICCI-CAL and COPAT was −0.4 (−1.9 to 1.1) l min with a percentage error of 29%. The mean of the differences between COCNAP and COPAT was 1.0 (−1.8 to 3.8) l min with a percentage error of 53%. The mean of the differences between COCNAP-CAL and COPAT was −0.2 (−2.0 to 1.6) l min with a percentage error of 35%. CONCLUSION: The agreement between CONICCI and COPAT is not clinically acceptable. TRIAL REGISTRATION: The study was registered in the German Clinical Trial Register (DRKS00023189) after inclusion of the first patient on October 2, 2020.