An outbreak of herpes simplex virus (HSV) type I gingivostomatitis occurred in a dental hygiene practice in November 1981. An epidemiologic investigation disclosed that 20 of 46 patients seen by the dental hygienist during a four-day period had this illness, whereas none of 26 patients seen by the dentist alone became ill. One day after the outbreak, the hygienist was found to have a herpetic whitlow. Identical endonuclease restriction enzyme type I HSV was isolated from the left index finger of the hygienist and from nine of 16 patients. The nine patients with positive cultures all had at least a fourfold increase in complement fixing antibody titer. A similar significant increase in titer indicated another seven cases.