OBJECTIVE Infection with human parvovirus B19 is the most common cause of transient aplastic crisis in patients with chronic hemolytic anemia. Multiple reports of children with simultaneous B19 infection and thrombocytopenia as well as the known association between experimental B19 infection and thrombocytopenia prompted us to hypothesize that B19 may be associated with childhood idiopathic, or immune, thrombocytopenic purpura (ITP). Because there is a paucity of evidence regarding a viral etiology for ITP, we performed a comprehensive study to explore its possible relationship to B19 infection. METHODS Thirty-five previously healthy children with ITP were studied prospectively. Bone marrow and peripheral blood were analyzed for B19 DNA using the polymerase chain reaction (PCR). Serum was analyzed for anti-B19 immunoglobulin (Ig) M and IgG antibodies using a B19 VP1 antigen-based enzyme-linked immunosorbent assay. Fourteen healthy children served as controls for peripheral blood PCR and serologic analyses. RESULTS The presenting clinical and laboratory features of the study population were typical of classic ITP. Seventeen of the 35 patients (49%) had evidence of B19 DNA in the peripheral blood, bone marrow, or both. Six of 35 (17%) had anti-B19 IgM antibodies. Eight of 35 (23%) were anti-B19 IgG seropositive. The control group had no positive PCR or anti-B19 IgM specimens. CONCLUSIONS Our results suggest that infection with human parvovirus B19 may be associated with childhood ITP. More investigation is warranted regarding the role of PCR methodology and serologic detection methods in defining B19 pathobiology as it relates to ITP.