Four thousand forty-two healthy children and adolescents, ages 12 months to 17 years, were vaccinated with a single dose of live attenuated varicella vaccine (VARIVAX; Merck Sharp and Dohme Research Laboratories) containing approximately 1000 to 1625 plaque-forming units/dose during clinical trials conducted from 1987 to 1989. Clinical follow-up of vaccinees revealed that 2.1 and 2.4% of vaccinees developed modified cases of varicella in the first and second years, respectively, after vaccination. Most of those who developed varicella postvaccination had an attenuated illness, characterized by fewer lesions and a lower incidence of fever (greater than or equal to 100 degrees F, oral) than after natural infection. The likelihood of developing varicella postvaccination decreased (P less than 0.0001) as the 6-week postvaccination glycoprotein-based enzyme-linked immunosorbent assay titer increased. In addition the number of lesions in these cases tended to decrease (P = 0.07 for Year 1 and P = 0.02 for Year 2) as the 6-week glycoprotein-based enzyme-linked immunosorbent assay titer increased. Thus the 6-week postvaccination glycoprotein-based enzyme-linked immunosorbent assay titer can be used as a surrogate marker for protection from natural disease.