Background: Most viral infections are known to exert adverse effects on bone marrow function. However, certain viruses have recently been found to be therapeutically beneficial in the treatment of some malignant disorders. Methods and materials: A retrospective study was conducted at the Armed Forces Hospital, Riyadh, Saudi Arabia. The changes in the hemalological parameters following varicella-zoster virus (VZV) infection in patients with a variety of hematological disorders were compared with those in a control group having the same spectrum of disorders and treated in the same unit over the same period of time but never had VZV inspection. Both groups of patients received the same treatment protocols for their primary hematological disorders. Definitive treatment (DT) such as chemotherapy alone, anti-thymocyte globulin or bone marrow transplant was also employed in the management of patients belonging to both groups. Results: White blood cell counts, platelet counts and hemoglobin concentrations in the study group started to increase 40 d after chickenpox or herpes zoster infection and these increases lasted for periods as long as 1050 d. The changes in platelet counts were more pronounced than those in other hematological parameters. There was a significant difference (P < 0.0001) between the two groups of patients in the values of platelet counts achieved between 280 and 1050 d after DT (mean platelet count: 262 x 10 9 /L in the study group vs. 180 x 10 9 /L in the control group, median: 288.17 x 10 9 /L in the study group vs. 180 x 10 9 /L in the control group, range: 102 to 415 x 10 9 /L in the study group vs. 26 to 365 x 10 9 /L in the control group of patients). Compared to the control group, the study group of patients achieved their maximum blood counts much earlier after DT. The maximum leucocytic count was achieved at a mean duration of 269.21 d in the study group and 349.61 d in the control group. The maximum hemoglobin level was achieved at a mean duration of 319.5 d in the study group and 402.6 d in the control group. The maximum platelet count was achieved at a mean duration of 271.4 d in the study group and 318.9 d in the control group of patients. Conclusion: VZV may behave differently from other members of the herpes group of viruses e.g. human cytomegalovirus and Epstein-Barr virus. Our observations suggest that VZV infection causes stimulation of bone marrow activity.