A 7-year-old boy had erythema infectiosum with typical manifestations. Over more than 40 days, these lesions showed no sign of regression, and chronic anemia became progressively more severe. Electron microscopic examination performed 20 days after onset showed abnormal Sezary-like lymphocytes. Bone marrow biopsy, which was performed to evaluate the anemia, was consistent with acute lymphocytic leukemia. Persistent parvovirus B19 infection may be connected with immunosuppression. Therefore, early electron microscopic study and bone marrow biopsy may be helpful for early diagnosis of hematologic malignancies.