Varicella-zoster virus (VZV) encodes at least six genes that are expressed during latency. One of the genes, ORF4, encodes an immediate-early protein that is present in the virion tegument. ORF4 RNA and protein have been detected in latently infected human ganglia. We have constructed a VZV mutant deleted for ORF4 and have shown that the gene is essential for replication in vitro. The ORF4 mutant virus could be propagated when grown in cells infected with baculovirus expressing the ORF4 protein under the human cytomegalovirus immediate-early promoter. In contrast, the VZV ORF4 deletion mutant could not be complemented in cells expressing herpes simplex virus type 1 (HSV-1) ICP27, the homolog of ORF4. Cells infected with baculovirus expressing ORF4 did not complement an HSV-1 ICP27 deletion mutant. VZV-infected cotton rats have been used as a model for latency; viral DNA and latency-associated transcripts are expressed in dorsal root ganglia 1 month or more after experimental infection. Cotton rats inoculated with VZV lacking ORF4 showed reduced frequency of latency compared to animals infected with the parental or ORF4-rescued virus. Thus, in addition to VZV ORF63, which was previously shown to be critical for efficient establishment of latency, ORF4 is also important for latent infection.