OBJECTIVE: Antibodies to the 70-kd heat shock protein were evaluated as biomarkers for cytomegalovirus infection. STUDY DESIGN: Fetal sera that was obtained by cordocentesis at 22 to 25 weeks of gestation from 53 mothers with a confirmed primary cytomegalovirus infection and 28 mothers (control group) without cytomegalovirus infection who had been screened for Rh incompatibility, were assayed by enzyme-linked immunosorbent assay for immunoglobulin M and G anti-70-kd heat shock protein antibodies. RESULTS: Eighteen of 53 fetuses (34.0%) from cytomegalovirus-positive mothers were infected, which was determined by culture or polymerase chain reaction. Anti-70-kd heat shock protein immunoglobulin M was detected in 17 of 18 sera (94.4%) from cytomegalovirus-infected fetuses, in 3 of 35 sera (8.6%) from uninfected fetuses, and in 0 of 28 sera from the control group (P <.0001). The level of antibody reactivity was correlated positively with fetal disease. Anti-70-kd heat shock protein immunoglobulin G was detected in sera from 52 of 53 fetuses (98.1%) from cytomegalovirus-infected mothers. CONCLUSION: The anti-70-kd heat shock protein immunoglobulin M assay may be of value in the assessment of fetal cytomegalovirus infection in pregnant women with a primary cytomegalovirus infection.