Varicella vaccine was first available in Taiwan in 1997. The aims of this study were to investigate varicella vaccine safety and occurrence of breakthrough in Taiwan during the first 3 years. The adverse events, incidence of breakthrough, and factors associated with breakthrough were analyzed.A personal interview using a structured questionnaire was conducted for the parents of 1248 children less than 12 years old who were vaccinated between 1998 and 2000. Incidence of adverse events and breakthrough were presented and factors associated with breakthrough were estimated by logistic regression.There were 27 (2.16%) breakthrough cases occurring during the maximum follow up period of 31 months, including 22 very mild or mild cases, 3 moderately severe cases, and 2 severe cases. Compared with those who did not have confirmed history of varicella exposure after vaccination, children with such exposure were approximately 28 times as likely to have breakthrough varicella develop (adjusted odds ratio = 27.75, 95% confidence interval: 6.12-125.78, P =.00). There were 91 (7.3%) reported cases of adverse events, including rash, fever, and pain or swelling, occurring within 2 weeks of vaccination.Although rare adverse events cannot be well-quantified in this study, the results suggest that, at least in the short term, varicella vaccine is well-tolerated and effective in Taiwan. Long-term monitoring program is necessary to ensure the safety of this vaccine.