Mycoplasma pneumoniae infection is a complex clinical problem. Atypical pneumonia with mild course, moderately elevated body temperature and presence of small auscultatory changes of the lung fields are its most common manifestation. Moreover, the severity of the symptoms is related to the age of patient. In the group of older children, disease may occur as interstitial or lobar pneumonia, less often as a pleurisy. Additionally fever can show higher values. Dry cough that lasts for 4 to 6 weeks is common and the most characteristic symptom of pneumonia caused by M. pneumoniae. Beyond the respiratory system, M. pneumoniae infection can manifest symptoms on almost every organ of the human body. For this reason neurological, cardiovascular, digestive, osteoarticular and skin complications are observed. The bacteria’s feature which results multitude of clinical manifestation is that despite of direct damaging of epithetical cell, has ability to stimulate autoimmune and prothrombotic processes. Diagnosis of infection is mainly based on serological methods. Treatment options include macrolides and therapy lasts at least 2 weeks. This article presents two cases of M. pneumoniae infection which are greatly different in their clinical course. In the first case the main manifestation of infection was pneumonia, which proceeded with a high fever, pleurisy, and the presence of many auscultatory changes of the lung fields. Additionally in this case were observed the symptoms of the gastrointestinal system in the form of vomiting and intense abdominal pain. On the other hand, the second case showed development of massive of deep vein thrombosis next to fresh M. pneumoniae infection and other risk factors.