Complications in two varicella zoster virus (VZV) vaccinated children with leukemia in remission are reported. Case I presented with varicella on day 30 after vaccination, with a relapse on day 49 and development of zoster on day 70. VZV was detected in vesicles by PCR on days 49 and 70. Case II presented with varicella on day 32 after vaccination, and VZV was detected in vesicles and nasal secretion. The manifestations were mild and responded to treatment. PCR methods were directed toward the R5 and PS regions. The virus from the two children was unambiguously identified as the Oka vaccine strain. The majority of Danish field strains had only one copy of the 112 basepair repeat element in the R5 region, but two, four and presumably higher copy numbers were also seen. All Danish field strains had the Pst1 cleavage site in the PS region.