Objectives: The aim of this study was to determine the frequency of SPN's etiology and diagnostic methods, CT puncture and exploratory thoracotomy used in their detection, the frequency of surgical interventions used in their removal as well as differences in age and gender in relation to the etiology and operability of the patients. Study design: The study is structured as a historical cohort study. Patients and methods: The case group consisted of 72 patients who were admitted to the Institute of Thoracic, Plastic and Reconstructive Surgery, Clinical Hospital Centre in Osijek, with diagnosed SPN. 54 patients (75 %) underwent an exploratory thoracotomy and 18 (25 %) underwent CT-guided needle puncture. 7 patients (9.7 %) were younger than 45 years of age and 65 (90.3 %) were older than 45 years of age. The study was conducted using data obtained from the patients’ medical records. Results: An increase was observed in the share of preformed exploratory thoracotomies in relation to the total number of thoracotomies, from 18.8 % in 2010 to 25% in 2014. Adenocarcinoma was the most common cause of malignant SPN’s and was detected in 14 patients (25.9 %) during exploratory thoracotomy, and squamous cell carcinoma was discovered in 7 patients (38.9 %) by CT-guided needle puncture. The most common surgical procedure performed was lobectomy, carried out in 15 patients (30.6 %). There was no statistically significant difference with regard to the gender, operability and etiology of SPN's between patients younger than 45 years of age and patients older than 45 years of age. Also, there was no statistically significant difference in the established etiology of SPN's diagnosed using CT-guided needle puncture and those diagnosed using exploratory thoracotomy. Conclusion: Additional research that will compare the data collected in all Clinical Hospital Centres in Croatia is necessary, to verify the incidence and etiology of SPN's for the entire country.