The incidence of neonatal herpes simplex virus (HSV) infection in King County, Washington, has increased progressively from a rate of 2.6 per 100,000 live births to King County residents during the years 1966 through 1969 to 4.8, 7.4, and 11.9 cases per 100,000 live births for the years 1970 through 1973, 1974 through 1977, and 1978 through 1981, respectively. Detailed interviews of the parents of newborns with neonatal HSV-2 infection revealed clinically symptomatic genital herpes in the mother or father in 65% of the parents and serological and/or cultural evidence of HSV-2 infection in the mother in the other 35% of cases. Clinically symptomatic first episodes of genital herpes during pregnancy occurred in four of the mothers who were delivered of newborns with neonatal HSV-2 infection; two of the mothers were symptomatic at the time of birth. The other 16 interviewed mothers who delivered newborns with neonatal HSV-2 infection all had serological and/or cultural evidence of past HSV-2 infection but were asymptomatic at term. All six newborns with neonatal HSV-1 infection acquired the disease from nongenital sources, and two newborns seemed to have acquired the disease from nonmaternal sources. These data indicate that detailed interviews of the parents are often useful in increasing the clinician's index of suspicion for neonatal HSV infection. The increased prevalence of genital HSV infection in the United States seems to be associated with a concomitant increase in neonatal HSV infection.