We identified two random samples of 216 primary care physicians each. In one sample, we made weekly telephone contact for active hepatitis A (HA) surveillance; in the other, we made no such contact (passive surveillance). Appropriate county health departments were notified whenever we identified a HA case by active surveillance. Active surveillance was associated with a 2.8-fold increase in reported HA cases compared to passive surveillance. The estimated benefit: cost ratio active/passive surveillance was 2.5:1.