We surveyed outbreaks of varicella zoster virus (VZV) and herpes zoster virus, involving 31 outbreaks of chicken pox, in a semiclosed institution in Osaka Japan during the 34 years between 1949 and 1984. Eight hundred forty-nine infants and children who had had clinical varicella during the first 4 years of life and those who had resided in the institution at least 12 to 144 months after the onset of varicella were included in the study. Nine cases of zoster were observed among children who had acquired varicella during the first year of life, but there was no case of zoster in those who had acquired varicella after 1 year of age. In 61,800 person-months of observation, the overall incidence rate of zoster was calculated as 0.15 per 1000 person-months for the population at risk. The incidence rate in children infected with VZV when younger than 2 months was 1.0 per 1000 person-months during the first decade of life. This rate was significantly (P less than 0.005) greater than that (0.19 per 1000 person-months) in children who had varicella when they were 2 to 11 months of age. These observations suggest that zoster occurs at a significantly shorter interval if VZV infection is acquired during infancy. More than 85% of subjects with prior infection were intimately reexposed to epidemic varicella during their residency in the institution, before having zoster. Epidemic reexposure to varicella during follow-up resulted in enhancement of preexisting immunologic reactivity, but did not prevent subsequent zoster in the population studied.