Thoracic electrical bioimpedance is a noninvasive, continuous method of obtaining cardiac output that requires no operator skill. However, the most recent thoracic electrical bioimpedance technology has not been validated in pregnancy. We therefore compared two methods of measuring cardiac output in pregnancy, thoracic electrical bioimpedance and thermodilution. We studied 11 patients who required pulmonary artery catheterization for peripartum management and measured cardiac output simultaneously by thoracic electrical bioimpedance and thermodilution. Among eight of nine patients, there was agreement (within +/- 20%) between the two methods. Bivariate linear regression with these nine cases showed excellent correlation (r = 0.91, p less than 0.001) with a slope of 1.04, which indicated a one-to-one relationship between thoracic electrical bioimpedance and thermodilution. The remaining two cases were removed from analysis because of septic shock in one case (which invalidates thoracic electrical bioimpedance) and 4+ tricuspid regurgitation in another case (which invalidates thermodilution). These data support that thoracic electrical bioimpedance measurement of cardiac output may be valid in most peripartum patients.