OBJECTIVE: To describe a cardiac output measurement using a new method to derive and analyze the long-axis ballistocardiogram that is less invasive than pulmonary artery thermodilution. DESIGN: Prospective physiologic study. SETTING: Intensive care unit of The Halifax Infirmary, a teaching hospital of Dalhousie University, Halifax, NS. PATIENTS: Thirty-nine patients in sinus rhythm with pulmonary artery thermodilution catheters or radial artery catheters in place. The first 30 subjects were the "learning set" and the next 9 were the "test set." INTERVENTIONS: A small (54-g) accelerometer was taped on the patient's chest. OUTCOME MEASURES: Measurements of time and amplitude coordinates of the acceleration and radial artery pressure wavepeaks, as well as anthropometric information. RESULTS: A stroke volume prediction equation was generated (R2 = 0.76) from the learning set. This equation was applied to the test set and correlated with the pulmonary artery thermodilution-derived stroke volumes (R = 0.79). Stroke volumes were compared using a previously described statistical method: a) bias (predicted > thermodilution) = 0.03 mL (95% confidence interval [CI] -4.2 to 4.8 mL); b) lower limit of agreement = -21 mL (95% CI -29 to -13 mL); c) upper limit of agreement = 22 mL (95% CI 14 to 29 mL). Of derived stroke volumes, 82% were within 15 mL of pulmonary artery thermodilution-derived values. CONCLUSIONS: The sternal acceleration ballistocardiogram combined with hemodynamic and demographic data in a probabilistic model shows promise of providing a less invasive measure of cardiac output than thermodilution.