34 nurses who had no previous history of chickenpox and were seronegative to varicella zoster virus (VZV) were immunised with a live attenuated varicella vaccine (OKA-RIT strain) and followed up for up to 36 months. No major vaccine reactions were observed. At 5 months and at 1 year, 94% of the nurses had seroconverted but at 3 years only 64% had detectable antibody. Lymphocyte transformation to VZV antigen tested in seven nurses in whom antibody was no longer detectable gave positive stimulation indices. 2 of 13 vaccinated nurses who looked after children with chickenpox became infected but 1 of them had not seroconverted after vaccination. Chickenpox developed in 6 out of 7 unvaccinated seronegative nurses who looked after children with chickenpox. This study shows that the OKA-RIT strain VZV vaccine is safe and immunogenic in adults and may therefore be considered for use in susceptible health care workers. The investigations also show that both cell-mediated and antibody tests are needed for long-term assessment of immunity to chickenpox after vaccination.