We previously reported on 8 young patients with undifferentiated mono- and oligoarthritis whose synovial tissue tested positive for Parvovirus B19 DNA by PCR. Three of these patients remained clinically undifferentiated and suffered from progressive inflammatory disease despite conventional therapy and repeated synovectomy. Intravenous treatment with immunoglobulin (0.4 g/kg body weight over 5 days) resulted in marked improvement in 2 of the 3 patients, allowing them to avoid repeated synovectomy during follow-up periods of 7 and 10 months, respectively.