BACKGROUND: We compared cardiac output (CO) measurements by the non-invasive electrical velocimetry (Aesculon) monitor with the pulmonary artery catheter (PAC) thermodilution method in children. METHODS: CO values using the Aesculon monitor and PAC thermodilution were simultaneously recorded during cardiac catheterization in children. Measurements were performed under general anaesthesia. To compare, three consecutive measurements for each patient within 3 min were obtained. The means of the three values were compared using simple regression and Bland-Altman analysis. Data were presented as mean (sd). A mean percentage of <30% was defined to indicate clinical useful reliability of the Aesculon monitor. RESULTS: A total of 50 patients with a median (range) age of 7.5 (0.5-16.5) yr were enrolled in the study. Mean CO values were 3.7 (1.5) litre min(-1) (PAC thermodilution) and 3.1 (1.7) litre min(-1) (Aesculon) monitor). Analysis for CO measurement showed a good correlation between the two methods (r=0.894; P<0.0001). The bias between the two methods was 0.66 litre min(-1) with a precision of 1.49 litre min(-1). The mean percentage error for CO measurements was 48.9% for the Aesculon monitor when compared with PAC thermodilution. CONCLUSIONS: Electrical velocimetry using the Aesculon monitor did not provide reliable CO values when compared with PAC thermodilution. Whether the Aesculon monitor can be used as a CO trend monitor has to be assessed by further investigations in patients with changing haemodynamics.