In 2008, a 28-year-old woman consulted our hospital due to general fatigue. Her ALT level was within normal range but she was positive for hepatitis B surface antigen (HBsAg). Her ALT level was nearly within normal range thereafter and she was consistently positive for HBeAg. Later, it was proven that she was negative for HBsAg in 1999. She had been a sex worker in 2007-2008. Complete genome sequencing revealed that her HBV was genotype C. The present case may indicate that it is possible for acute infection with HBV genotype C to progress to chronic infection in adults.