Introduction. Primary multiple malignant tumors are the independent occurrence of two or more malignant neoplasms in one patient. The number of such patients registered in Russia is more than 288 thousand people, with an annual tendency for this figure to increase. Considering that the patient has two neoplasms, for which specialized care can be provided, including simultaneously, questions naturally arise - how to correctly formulate the final clinical diagnosis and correctly select the diagnosis code. Patient F., 68 years old, with rectal cancer T3N0M0, II (adenocarcinoma) applied to the hospital for diagnosis and determination of treatment tactics. Cysts were diagnosed in both kidneys. In the right kidney 4.3 cm (Bosniak I); in the left kidney a complex cyst 7.7 cm with a hemorrhagic component (Bosniak II). The patient underwent rectal resection, which was complicated by perforation of the hollow organ, sepsis of a multidrug-resistant strain of the pathogen, which led to the death of the patient. Clinical diagnosis - C20 rectal cancer T3N0M0, stage II. The autopsy revealed a discrepancy diagnoses (category II) due to the presence of an undiagnosed second tumor – cancer of the left kidney. Pathological diagnosis: C97 Primary multiple neoplasms: 1) Rectal cancer pT3N0M0. 2) Left kidney cancer, pT1aN0M0. Conclusion. Before the 2022 Society of Pathologists guidelines, the primary diagnosis of multiple tumors was considered by pathologists as a single diagnosis. All of the patient's cancers were the main ones. Since 2022 the use of code C97 has been abolished, and the main diagnosis is now the leading, most severe neoplasm. This requirement is set out in a small number of clinical guidelines and registers, which does not correspond to the requirements of the ICD-10 classification itself, which is in effect in Russia. The country is gradually abandoning the use of the C97 code. Numerous issues related to the choice of diagnosis code, as well as the interpretation of outdated provisions of the ICD-10 classification, remain unanswered, due to the fact that at present there are no unified recommendations for clinical diagnosis construction. The axiom that clinical and pathological diagnosis are one and the same is also currently being challenged.