This article describes a case of lung cancer, the symptoms of which have long been regarded as manifestations of pneumonia; the stages of diagnostic search from the first symptoms of cancer to radical treatment are revealed. At the same time, the patient was diagnosed with monoclonal gammopathy of unknown significance, which at the stage of differential diagnosis suggested the development of synchronous cancer – lung cancer and the onset of multiple myeloma (MM). A number of literature sources were analyzed, according to the results of the analysis, it was found that at the moment six cases of the development of synchronous lung cancer and ММ have been described in the world, in most of which the age of patients was less than 70 years, which indicates a relatively early development of ММ. The issues of differential diagnosis of pneumonia and lung cancer are considered; the importance of early detection of malignant neoplasms (especially at the outpatient stage) is emphasized. It has been established that in some cases pneumonia can mask cancer, especially the detection of neoplasia after diagnosing pneumonia of the upper lobe localization. It has been suggested that the pathogenesis of lung cancer and ММ associated with intensive expression of CD38 may be common, and an example of a positive clinical effect on unresectable lung cancer with daratumumab is given (CD38-blocking monoclonal antibody used in MM therapy). At the same time, the revealed observation of the combined development of oncological diseases is very interesting, since in patients with different types of cancer, as a rule, each tumor tends to develop sequentially. However, our case was analyzed in comparison with literature data with cases of concomitant development of multiple myeloma and lung cancer simultaneously. At the same time, common genes and related pathogenesis of progression of two types of tumors were identified due to activation of the expression of the mitochondrial trans-2-enoyl-CoA reductase (MECR) concentrator gene.