Viremia in parainfluenza was studied in 80 patients (35 children and 45 adults) in the time course of the disease during the springautumn period of 1969-1974. For the detection of viremia, the immunofluorescence procedure, namely, the detection of early stages of virus replication in the tissue culture cells infected with the blood from the patients, was used. The tests were performed with 204 blood specimens. The blood clot was diluted 1 : 1 with distilled water and followed the conventional method. Commercial FITZ-globulins with high antibody titres to parainfluenza virus type 1, 2, and 3, influenza A2 and B and adenovirus types 3 and 7, were used. The clinical manifestations of parainfluenza in the observed patients were typical of the disease. Complications developed in 60% of the cases. As a result of clinical and laboratory examinations, viremed early and late (up to 22 days) in the disease in patients of different age groups both at high and low fever; for longer periods and later in the disease it was observed in patients with complications (pneumonia, maxillary simusitis), but always in the presence of clinical manifestations of parainfluenza.