Eight hundred healthy seronegative children and 32 adults were vaccinated with the Oka-strain live attenuated varicella vaccine (Varilrix), manufactured by Smith Kline-RIT, Belgium. The vaccine was safe (no side effects) in healthy individuals, highly immunogenic (dose-dependent induction of humoral antibodies and specific cell-mediated immune response), and highly protective against clinical varicella. The simultaneous administration of a measles-mumps-rubella vaccine together with the varicella vaccine is possible, although combination in the same syringe was less effective than simultaneous administration in different arms. Various observations suggest that the risk of contracting zoster in adulthood may well be reduced for varicella vaccinees. Based on our results, it is recommended that seronegative hospital staff be vaccinated. Furthermore, if the varicella vaccine should prove to be cost effective, large-scale immunization of healthy children may be indicated.