Serum concentrations of alpha-fetoprotein (AFP), variably elevated during liver injury, have been suggested to be of prognostic importance in acute liver failure (ALF), higher values being associated with improved outcome. Using a nephelometric assay, we measured AFP in sera obtained on admission from 206 patients prospectively enrolled in the US ALF Study, and on day 3 in 162 of these patients. The AFP ratio was defined as the day 3 AFP concentration divided by that observed on day 1. Median (range) admission serum AFP in all patients was 8.1 (1-1,811) ng/mL and increased to 17.6 (1.1-1,162) ng/mL on day 3 (P < 0.001). Higher absolute levels were not associated with improved outcome. In fact, admission AFP levels were lower in survivors not receiving transplants than in those who died or were transplanted (P < 0.001), whereas there was no difference between the 2 groups on day 3 (P = 0.34). However, a rise in AFP values between day 1 and day 3 indicated a better prognosis: the AFP ratio was 2.2 (0.11-22.1) in spontaneous survivors and 0.87 (0.11-16.4) in nonsurvivors (P < 0.001). An increasing AFP level indicated by an AFP ratio ≥1 was observed in 70 of 98 (71%) survivors, whereas a ratio <1 was observed in 51 of 64 (80%) nonsurvivors. In conclusion, AFP values change dynamically during ALF. In this large prospective study, higher absolute values of AFP did not predict a favorable outcome, but a rising level of AFP over the first 3 hospital days frequently indicated survival.