Abstract.A serological investigation on infants with incidentally detected cytomegalovirus (CMV) excretion was made in an attempt to differentiate between congenital and acquired infections. Generally, each of the patients, 0–12 months old at the detection of CMV‐excretion, was studied by indirect immunofluorescence (IIF) test for IgM‐antibodies in cord serum and by complement fixation (CF) test performed on cord serum and a number of sera drawn after the detection of CMV. Two out of 4 patients with virologically confirmed congenital CMV‐infection, as shown by positive virus isolation within 1 week of age, had a positive CMV‐IgM‐test in cord serum. One of these two children also had a persistently high CMV‐CF‐titer from birth until 2 months of age, indicating congenital infection. In the remaining 46 cases, all with the CMV‐excretion detected after 3 weeks of age, in half of the cases after 5 months, no positive IgM‐reaction was recorded in cord serum. No persistently high CF‐titer could be demonstrated among 13 out of the 46 patients from whom sera were drawn at birth and at 1–4 months. Six out of these 13 patients had a CF‐titer rise after the period of 1–4 months, indicating acquired infection. However, also one of the congenitally infected children had a similar titer increase. Many patients lacked characteristic serological patterns, some of them in spite of access to sera drawn at birth, as well as at 1–4 months of age and later on. It could be concluded that the possibility of making a serological distinction of congenital and acquired infant CMV infection found, by chan