Acute lower respiratory tract infections are one of the major causes of childhood morbidity and mortality in developing countries. This study was undertaken at Hacettepe University Children's Hospital to determine the role of viruses and Mycoplasma pneumoniae in respiratory tract infections in children. Eighty-three patients with lower respiratory tract infections were selected at random from among the children admitted to the hospital for evaluation of respiratory symptoms. Acute and convalescent serum samples were collected from all patients for the complement-fixation test and the following antigens were used: respiratory syncytial virus (RSV), adenovirus, parainfluenza virus Type 1, influenza viruses A and B, and Mycoplasma pneumoniae. The test was positive in 39 of 83 patients (47%), and RSV was the most frequent agent detected serologically (15.7%).