The pathogenic influence of viral agents in chronic inflammatory joint diseases like rheumatoid arthritis has been discussed for many years. More recently, DNA of several viruses, among them parvovirus B19 (B19), was traceable by PCR analysis in synovial fluid and synovial tissue. To investigate the potential role of parvovirus B19 in rheumatoid arthritis, we analyzed the expression of B19 VP1/VP2 proteins by immunohistochemistry in paraffin sections of 63 synovial specimens in rheumatoid arthritis (RA; n = 29), psoriatic arthritis (PSA; n = 6), nonspecific arthritis or synovitis (n = 26), and normal synovia (n = 2). Thereby we could demonstrate replicative virus infection in a variable number of cells in about 90% of rheumatoid specimens and in four of six (66%) cases of psoriatic arthritis, but only in 38% of cases with chronic reactive inflammation and one case of normal synovia. In virus-positive rheumatoid specimens, moreover, the average number of affected cells was significantly higher than in virus-expressing synovia of nonspecific reactive inflammation. These findings support the importance of B19-viral infection in the pathogenesis of chronic arthritis. B19-positive cells in the synovia could be ascribed to CD20- or CD3-positive B- or T-lymphocytes by double immunostaining. Based on these results, B19 infection of lymphocytic cells also seems possible.