Chickenpox is a relatively mild disease in healthy children but may be life threatening in immuno-suppressed patents, neonates, and normal adults, especially smokers-for whom the risk of varicella pneumonia is high. The epidemiology of chickenpox appears to be changing: There has been an unexplained upward shift in the age distribution of cases over the last 20 years. This is reflected by increased consultations for chickenpox in general practices and more deaths in England and Wales. On the basis of hospital admissions for chickenpox in young adults, there is evidence of a similar trend in the United States. This epidemiologic change has important consequences for future mortality rates and for risk of infection in health care workers and pregnant women. The potential use of the varicella vaccine should be considered as a measure to reduce the risk of nosocomial transmission in view of the possible changing epidemiology of varicella.