An outbreak of hepatitis B in southeastern Connecticut was traced to an oral surgeon. In a serosurvey of his 754 noninstitutionalized patients, 511 (68%) participated. The rates of seropositivity for hepatitis B surface antigen (HBsAg) or antibody to HBsAg by year of oral surgery were 4.8% in 1977, 7.4% in 1978, and 14.8% in 1979. The transmission of hepatitis B, as measured by seropositivity, occurred between November 1978 and August 1979; the peak transmission was in February (31.2% seropositivity). Seropositivity was strongly correlated with the extent of surgical trauma (P less than 0.0001). HBsAg subtyping revealed that the oral surgeon and all four subtypable patients demonstrated both d and y reactivity, as did the patient from whom it was most likely the oral surgeon acquired his infection: a mentally retarded patient who had 24 extraction in March 1978. This study demonstrates that the transmission of hepatitis B between dental practitioners and their patients frequently results in subclinical infections.