The names of collaborators may be found on page 576Objective To compare the clinical manifestations and laboratory abnormalities associated with primary cytomegalovirus (CMV) infection in pregnancy with recurrent and non-active CMV infection (controls). Design A prospective cohort study.Setting Rome, Latium and other Italian regions.Population Three hundred and sixteen pregnant women with CMV infection: 102 had primary infection, 105had recurrent infection and 109 with non-active infection were followed up as controls. Methods CMV diagnosis was based on serological examinations (CMV IgG, IgM and IgG avidity) and detection of CMV DNA by polymerase chain reaction in maternal serum, urine and cervical samples. The clinical history and laboratory evaluations were carried out at enrolment and at each subsequent visit, every one to three months. Main outcome measures Identification of clinical and laboratory indicators of primary CMV infection in pregnancy. Results Compared with women with recurrent or non-active infection, women with primary infection had a statistically significant higher prevalence of fever, asthenia, myalgia and flu-like syndrome ( P < 0.001). In particular, relevant symptomatology was observed in 32 women (31.4%), of whom 25 had flu-like syndrome and 7 persistent fever as a single manifestation. Moreover, women with primary infection showed a significantly increased rate of lymphocytes !40% (39.2% vs 5.7% or 3.7%, respectively, P < 0.001) and elevated aspartate aminotransferase and/or alanine aminotransferase levels (35.3% vs 3.9% or 0.9%, respectively, P < 0.001): lymphocytosis and/or increased aminotransferases occurred in 53 patients (52%). In total, clinical manifestations and/or laboratory abnormalities occurred in 61 women with primary infection (59.8%) compared with 20 with recurrent infection (19%) and 13 controls (11.9%) ( P < 0.001). Conclusion Clinical manifestations (i.e. flu-like syndrome, fever) and abnormal laboratory findings (i.e. lymphocytes !40%, elevated aminotransferases) may suggest the presence of primary CMV infection and should prompt subsequent virological investigations.