Vein thrombosis of the lower extremities, especially deep, is an acute condition which develops under the influence of multiple factors. In this case, the immediate threat to the patient’s life is associated with the development of acute pulmonary embolism (PE). A wide range of symptoms accompanied by the underlying disease manifestations determines patient's grave condition, complicates timely diagnosis and treatment, contributes to a poor prognosis and high mortality. Thus, the risk ratio of PE development should be assessed in all patients, regardless of age. The article provides data on the epidemiology, risk factors, clinical features and diagnosis of thromboembolic complications in real clinical practice. The paper describes a clinical case of community-acquired pneumonia in a young man with asymptomatic deep vein thrombosis of the lower extremities and the development of multiple pulmonary embolism. A combination of diseases determined the severity of patient’s state, the challenges of intravital diagnosis of the concomitant pathology and a poor prognosis due to such a combination. The diagnosis was confirmed at autopsy by the detection of various clots in the lung vascular bed.