BACKGROUND: Sexual transmission of hepatitis C virus (HCV) can occur, albeit inefficiently, and this represents a possible cause of community- acquired infections. This study describes a case of asymptomatic HCV infection acquired by a repeat blood donor from her sexual partner. CASE REPORT: A female repeat blood donor showed anti-HCV seroconversion and a slight elevation in alanine aminotransferase. She had a normal physical examination and no clinical symptoms. She admitted a sexual partnership with a man with chronic HCV infection. Genotyping showed subtype 3a infection in both. Nucleotide sequence analysis of the hypervariable region of the viral envelope was performed on five clones obtained from the donor and the partner. Five blood donors with subtype 3a infection were analyzed as controls. The mean homology among clones was 99.3 percent (95% CI, 98.9–99.7) in the donor and 96.8 percent (95% CI, 94.4–99.2) in the partner, which suggests a more recent infection in the woman. The mean homology between donor and partner was 93.4 percent (95% CI, 93.1–93.8), which is different from that between donor and controls (76.2%; 95% CI, 73.3–79.1; difference between means, 17.2%; 95% CI, 16.0–18.4). This suggests that the infection was transmitted to the donor from her sexual partner. Sexual intercourse is the most probable route of transmission, because parenteral risk factors were absent. CONCLUSION: Heterosexual transmission of HCV can occur in the absence of a long-lasting contact, and the infection can be asymptomatic. It remains to be determined whether the sexual partners of HCV-infected subjects should be deferred from blood donation.