Intrauterine infection of human parvovirus B19 has recently been identified as an etiology for nonimmune hydrops fetalis (NIHF) and fetal death. To examine the frequency of B19 infection in cases of NIHF, forty two cases of NIHF of unknown cause were tested for the presence of B19 antibodies by enzyme-linked immunosorbent assay (ELISA) and B19 DNA by polymerase chain reaction (PCR). Three maternal sera were positive for B19 IgM antibody and one fetal serum was positive for B19 DNA. Overall, four (10%) of the 42 cases were positive for B19 infection. All of the four cases positive for B19 infection were found during or just before an outbreak of erythema infectiosum and the incidence was calculated at 36% if only cases found during the outbreak were collected. In all four cases of B19 infection, NIHF was diagnosed at 20 to 23 weeks of gestation, which suggested that B19 infection might be a particular threat to the fetus during this stage of gestation. In conclusion, B19 infection may contribute to 10% of cause unknown NIHF and the incidence may be higher if cases during outbreaks are exclusively collected.