For mothers with chronic hepatitis B virus (HBV) infection, the Centers for Disease Control and Prevention recommends immunoprophylaxis to decrease perinatal transmission. However, its effectiveness and risk factors for failure have not been well-studied in community practice. To investigate the effectiveness of a contemporary immunoprophylaxis protocol. Observational study. An HBV perinatal immunoprophylaxis program within Kaiser Permanente Northern California. 4446 infants born to 3253 HBV-positive mothers between 1997 and 2010. Adherence to immunoprophylaxis, follow-up testing rates, maternal risk factors for HBV transmission, and transmission rates. The infant infection rate was 0.75 per 100 births from 1997 to 2010 (Poisson 95% CI, 0.48 to 1.10). Rates per 100 births were 3.37 (CI, 2.08 to 5.14) for e antigen-positive mothers and 0.04 (CI, 0.001 to 0.24) for e antigen-negative mothers. Among mothers with viral load testing, the lowest level associated with transmission was 6.32 × 107 IU/mL. Infection rates per 100 births were 3.61 (CI, 0.75 to 10.56) among the 83 births to mothers with viral loads of 5 × 107 IU/mL or greater and 0 among the 831 births to mothers with viral loads less than 5 × 107 IU/mL, regardless of e antigen status. Testing for HBV immunity and infection was less complete in earlier years. Viral load testing was only consistently available starting in 2007. Prenatal HBV screening followed by postnatal prophylaxis is highly effective in preventing vertical transmission of HBV. A negative e antigen status or a viral load less than 5 × 107 IU/mL (90.9% of women tested) identifies women at extremely low risk for transmission after immunoprophylaxis who are unlikely to benefit from further interventions. Kaiser Permanente Community Benefit and National Institutes of Health.