Introduction: Influenza is an acute respiratory disease caused by Influenza virus A and Influenza virus B. The disease affects the upper and/or lower respiratory tracts and is often accompanied by specific signs and symptoms such as fever, headaches, myalgia and weakness. Study design: Cross-sectional study. Research objectives: Showing the clinical manifestations and laboratory specificities of the 2017/2018 seasonal influenza in patients as well as pointing out specific markers of infection which could allow us to unambiguously define the disease diagnosis. Participants and methods: The study included 302 patients aged 18 or older, of both sexes, with laboratory-confirmed influenza diagnoses. The patients were treated at the University Hospital for Infectious Diseases “Dr. Fran Mihaljevic” in Zagreb between November 2017 and April 2018. The data on patients was collected from medical records. The study excluded patients who did not have laboratory testing done during their first check-up at the University Hospital and patients who did not have a complete medical record. Results: Statistical data analysis determined the frequency of specific influenza virus subtypes, and the clinical symptoms and results of laboratory testing were shown and compared according to age, sex and influenza type. Conclusion: Analysing the obtained data determined that in the 2017/2018 influenza season virus B/Yamagata was proven in 90.2% of patients. It was determined that there is no statistically significant difference in the average number of complications and symptoms in patients with and without chronic conditions. Most patients’ leucocyte levels were within the reference range, although cases of neutrophilia and lymphopenia were present. Transaminase levels were elevated in a higher number of men than women, and more often found in patients under 65 years of age. Almost all patients had elevated CRP, fibrinogen and D – dimer levels.