Hepatitis B virus (HBV) infection almost always recurs after liver transplantation in patients who were surface antigen (HBsAg) positive before surgery but apparent de novo acquisition of infection in a transplant setting has not previously been reported. We have used sensitive techniques to elucidate the origin of such infections in patients in a California transplantation programme. We tested post-transplant serum from 207 patients who had been HBsAg negative and found 20 to be HBsAg positive. The origin of infection was identified in 7 patients, being occult pre-transplant infection in 5 and occult infection in the donor in 2. No pre-transplant patient nor donor with demonstrable HBV DNA had serological markers of hepatitis B. Post-transplant HBV DNA was present in serum from 19 patients. Analysis of the variable pre-S region of HBV demonstrated 100% sequence homology between recipient liver and post-transplant serum (2 patients) and between donor serum and recipient post-transplant serum (2). There was only 84% homology between the 2 different patients infected with subtype adw. 19 patients are alive, 9 without histological evidence of hepatitis (mean follow-up 33 months), and survival was significantly greater than that of a group with recurrent HBV infection. Apparent acquisition of HBV infection with liver transplantation is not rare, and may be due to occult pre-transplant infection or occult infection in the donor. The post-transplant outcome of this infection tends to be benign but our findings do underscore the clinical relevance of HBV infection in the absence of serological markers.