In 1998, 3 years after vaccine licensure, child care centers (CCC) in Los Angeles County continued to report varicella outbreaks. We investigated outbreaks at 2 CCCs to determine the cause for them, such as low vaccination coverage levels or unexpected low vaccine effectiveness. We collected information on past history of varicella, illness during the outbreak, and prior varicella vaccination among CCC attendees. We found that CCC "H" had a vaccination coverage of 87% (34/39) compared with 30% (6/20) in CCC "L." The overall attack rate was lower in CCC "H" (31%) than in "L" (61%; P value =.03). Vaccine effectiveness for any varicella was 71% in "H" and 100% in "L." Vaccinated children with varicella had milder disease than unvaccinated. In conclusion, we found varicella outbreaks in CCCs with both high and low vaccination coverage. Vaccine effectiveness was within the range predicted by the literature. Vaccination led to a lower attack rate in the highly vaccinated CCC and appeared to protect from severe disease.