Etiology and clinical manifestations have been studied in 153 adult patients with lower respiratory tract infection, and the results are presented according to clinical and radiographic diagnosis. Laboratory investigations revealed that bacterial infection, mycoplasma and chlamydia included, occurred as often in 22 patients whose clinical diagnoses of pneumonia were not evident radiographically, as in 20 patients with radiographic pneumonia. In the latter group significantly higher values of erythrocyte sedimentation rate and C-reactive protein were demonstrated. The most common pathogen was influenzavirus A, followed by respiratory syncytial virus, Streptococcus pneumoniae, and Mycoplasma pneumoniae. Chlamydia pneumoniae infection was found in 3 patients with radiographic pneumonia. The study supports the traditional view that patients with a positive chest radiograph as a rule present more serious manifestations of lower respiratory tract pathology than patients with a normal radiograph. However, as only 1/9 patients with pneumococcal infection and 2/7 with mycoplasmal infection had radiographic evidence of pneumonia, radiography alone did not seem to offer sufficient information for selecting patients for antibacterial therapy.