Analysing the X-ray picture of childhood pneumonias from 8 X-ray parameters chosen by the authors has ascertained that bacterial pneumonias are most characterized by 2 X-ray signs: total segmentation and homogenicity of the lesion, whereas viral pneumonias by 4-5 or more out of the 8 chosen signs, among which there is partial segmentation and unhomogeneity of the lesion with increased lung outline within the lesion focus. This enables at least 96% probability to differentiate viral and bacterial pneumonias. The differences between pneumonic changes in viral respiratory infections are not so well-defined. But at the same time the X-ray pattern of RS infection most commonly includes an obstructive component (most infrequently in influenza and parainfluenza), partial segmentation and a limited area of infiltration (only 1-2 segments are involved). Adenoviral infections are most frequently characterized by increased lung outline beyond the pneumonic focus, indicating concomitant bronchitis beyond the pneumonic area, as well as by the atelectasis and extensive infiltration (involving 3-5 segments). Thus, the X-ray technique enables etiological rapid diagnosis during the first three days of the onset of the disease, providing timely initiation of etiothropic therapy.