The aim of this pilot study is to evaluate the circulatory safety of treatment with the molecular adsorbents recirculating system (MARS) by determining the effect on systemic hemodynamics of a single MARS treatment in patients with acute-on-chronic liver failure (AOCLF). In eight patients admitted with AOCLF, a single 10-hour MARS treatment was performed. Systemic hemodynamic variables were determined before and during treatment. Bilirubin and urea were monitored as measures of protein-bound and water-soluble toxins. During MARS treatment, mean arterial pressure increased from 67 +/- 9 to 76 +/- 6 mm Hg (P < .05). Systemic vascular resistance index increased from 757 +/- 134 to 884 +/- 183 dyne x s/cm(5)/m(2) (P < .05), whereas cardiac index remained constant (5.9 +/- 0.7 v 6.0 +/- 1.1 L/min/m(2)). No episode of dialysis-induced hypotension was observed. Systemic oxygen consumption remained constant (92 +/- 30 v 93 +/- 11 mL/min/m(2)). Bilirubin levels decreased from 537 +/- 192 to 351 +/- 106 micromol/L (P < .05), and urea levels, from 19.1 +/- 13.9 to 6.7 +/- 5.1 mmol/L (P < .05). In conclusion, MARS treatment proved safe in critically ill patients with no attributing side effects.