Serum HBV DNA from infants and children with fulminant hepatitis B (FHB) or acute self-limiting hepatitis B (AHB) and patients believed to be contamination sources was amplified by polymerase chain reaction (PCR) followed by direct sequencing of the precore region. A precore mutation from G to A at nucleotide 1896 was found in 5 of 14 FHB patients and in 3 of 10 AHB patients. Among the 10 infants who developed hepatitis during the first 6 months of age, the precore mutation was detected in only 2 of 7 with FHB and in 1 of 3 with AHB. In 1 infant with FHB, a shift from wild type to precore mutant predominance occurred in the serum virus population during the incubation period. Thus, the precore mutation is neither necessary nor sufficient to cause FHB in childhood, although its contributory role cannot be excluded. Factors other than precore mutations may be important in the pathogenesis of FHB.