The objective was to estimate the probability of sporadic hepatitis B virus (HBV) and human immunodeficiency virus (HIV) transmission to a patient from an infected surgeon due to percutaneous injury during an invasive procedure. Risk was estimated based on a model involving three probabilities: A, the probability that the surgeon will sustain a percutaneous injury during an invasive procedure; B, the probability that the sharp object causing the injury and now contaminated with the surgeon's blood will contact the patient's wound; and C, the probability that infection would be transmitted to the patient after such an exposure. The probability of transmission during one procedure is p = A x B x C. The probability of transmission to at least one patient during N procedures is 1-(1-p)N. Values for A, B, and C were estimated from prospective studies. The estimated probability of transmission from an infected surgeon to a patient during a single procedure is 0.00024-0.0024% for HIV and 0.024-0.24% for HBV if the surgeon is positive for hepatitis B e antigen (HBeAg). The estimated probability of transmission to at least one patient during 3,500 procedures (estimated to be performed during an HIV-infected surgeon's remaining working life) is 0.81-8.1% for HIV; 57-100% for HBV if the surgeon is an HBeAg carrier. These estimates represent population averages and may not necessarily apply to a particular procedure performed by a particular surgeon, for which the risk may be considerably lower or higher than the estimated average. This risk assessment, which is based on limited data and does not take clusters of transmission into account, predicts that the risk of sporadic HBV transmission from infected surgeons to patients due to percutaneous injury during an invasive procedure is small and that the risk of HIV transmission is less than that for HBV. More data are needed to understand both sporadic and epidemic transmission in order to further reduce patient risk.