We described two cases having erythroid hypoplasia and pancytopenia, respectively, caused by human parvovirus B19 (PVB19) infection on chemotherapy. The first patient was a seven-year-old boy with Non-Hodgkin's lymphoma. He has obtained complete remission with LSA2L2 protocol, but, immediately after this remission, he suddenly developed high fever, erythema on cheeks and severe anemia without reticulocytes. We concluded that the cause of anemia and other symptoms were due to PVB19 infection because PVB19 DNA in his serum was detected by dot blot hybridization and polymerase chain reaction, although specific antibodies to PVB19 remained absent. He received gamma-globulin intravenously (200mg/kg/day for 5 days). The fever and erythema were improved promptly, but viremia and anemia lasted a few weeks and the specific antibodies to PVB19 were negative for about two months thereafter. The second patient was a nine-year-old boy with common ALL in remission. Pancytopenia suddenly occurred under maintenance therapy. Because IgM to PVB19 was detected in his serum, the patient was diagnosed to have temporary pancytopenia due to PVB19 infection. In conclusion, it is important to confirm specific antibodies and PVB19 DNA in serum when anemia or pancytopenia of unknown cause occurs in immunocompromised patients receiving chemotherapy.