Apart from transplantation, until very recently, there have been no life-saving therapies available for liver failure. Now, one first-generation liver assist device is on the market and others are progressing through both biologic and device clinical trials. Companies with liver assist devices sort into two main groups: companies with dialysis and ultrafiltration systems that improve upon kidney dialysis and fall squarely on the device side of the divide for regulatory purposes, and those that incorporate living cells in a device-biologic combination, which require drug-like approvals. Regardless of the approval process, all companies need compelling efficacy data to convince clinicians and payers of the benefits of a brand new therapy. But the large numbers of variables and unknowns concerning liver function and liver disease have posed considerable obstacles to designing prospective, randomized, controlled clinical trials-a problem highlighted by Circe Biomedical's halting of its large Phase II/III clinical trial for lack of efficacy. There is no question that there is a tremendous need for liver support. What is tantalizing for both companies and investors in this area is that they feel liver assist devices do work-the liver can recover, physiological functions improve, patients with a prognosis of death have survived. But they just haven't been able to prove it yet, not in terms of the only endpoint that really counts at this early stage in the field, improvements in 30-day survival rates.