Neuroendocrine tumors (NET) are tumors arising from neuroendocrine cells that form so-called diffuse neuroendocrine system located in the mucosa of various organs. They are mostly localized in the digestive and bronchopulmonary tracts and more often appear sporadically. NETs are associated with an increased incidence of developing other primary tumors (second primary malignancy or SPM), which can be divided, depending on the time of onset, into synchronous or metachronous. By definition, synchronous tumors develop within 6 months of NET diagnosis and metachronous tumors are those that arise > 6 months before or after NET diagnosis. In this thesis, we analyzed the archives of University Hospital Centre Zagreb in the period from 2000. to 2021., and recorded all patients with NET diagnosis. By analyzing the documentation, 201 patients with NET were found. Also, 41 second primary tumors were found in 37 (out of a total of 201) patients with NET. In 12 (29.3%) cases they were synchronous and in 29 (70.7%) cases metachronous. The most common localization of SPM were digestive (21 cases, 51.2%) and urogenital system (9 cases, 21.9%). To conclude, in a group of 201 patients with NET, the prevalence of synchronous and metachronous tumors was 17.9%, which indicates that the presence of SPM in patients with NET is not rare and should be taken into consideration during diagnosis, therapy and follow up.