The aim of this study was to investigate the prevalence and seasonal distribution of respiratory viruses in pediatric and adult outpatients and inpatients who were admitted to hospital with the symptoms of upper and lower respiratory tract infections, during a 12-year period. A total of 5102 clinical samples (4372 nasopharyngeal swabs, 316 bronchoalveolar lavages, 219 transtracheal aspirates, 163 nasopharyngeal aspirates, 20 sputum, 10 nasal swabs) examined in our laboratory between January 1st 2002 and July 17th 2014, were evaluated retrospectively. Of the specimens, 1107 (21.7%) were obtained from outpatients and 3995 (78.3%) from hospitalized patients. Of the patients, 2851 (55.9%) were male and 2251 (44.1%) were female, while 1233 (24.2%) were adults and 3869 (75.8%) were children (age range: 1 day - 93 years; median: 3 years). Respiratory samples were investigated for the presence of respiratory syncytial virus (RSV), influenza virus type A and B (INF-A, INF-B), adenovirus (AdV), parainfluenza viruses (PIV types 1-4), human rhinoviruses (HRV), human coronaviruses (HCoV), human metapneumovirus (HMPV) and human bocavirus (HBoV). All specimens were tested by both direct immunofluorescence antibody (DFA) and shell vial cell culture (SVCC) methods. In DFA assay the samples were initially screened by fluorescent-labeled polyclonal antibodies, and the positive ones were typed by using monoclonal antibodies (Light Diagnostics, Merck Millipore, USA). In SVCC, HEp-2, MDCK, A-549 and Vero cell lines were used for the isolation of viruses. In addition to these methods, real-time multiplex PCR methods (RealAccurate®, Respiratory RT PCR, PathoFinder, Netherlands and Seeplex® RV15 ACE Detection, Seegene, South Korea) were used for the detection of respiratory viruses in samples (n= 2104) obtained from 2007 to 2014. Respiratory viruses were detected in a total of 1705 (33.4%) patients, of them 967 (19%) were male and 738 (14.4%) were female. Three hundred and eighteen (18.6%) of the 1705 patients were infected with multiple respiratory viruses. The most frequently observed co-infections were RSV+INF-A (40/318; 12.6%), and RSV+PIV (33/318; 10.4%). The rate of positivity for the respiratory viruses in pediatric and adult groups were 35.4% (1369/3869) and 27.3% (336/1233), respectively (p< 0.000). The most frequently detected virus in pediatric group was RSV (336/1369; 24.5%), followed by influenza viruses (314/1369; 22.9%), PIV (197/1369; 14.4%), HRV (118/1369; 8.6%), AdV (75/1369; 5.5%) and the others (49/1369; 3.6%). On the other hand the most frequently detected virus in adult group was influenza viruses (181/336; 53.8%) followed by AdV (37/336; 11%), RSV (24/336; 7.1%), PIV (24/336; 7.1%), HRV (23/336; 6.8%) and the others (9/336; 2.7%). The rate of multiple virus infections in pediatric and adult groups were 7.2% (280/3869) and 3% (38/1233), respectively. Most of the coinfections (280/318; 88%) were detected in children. Respiratory viruses were detected positive in 40.2% (445/1107) of outpatients, and in 31.5% (1260/3995) of inpatients (p< 0.000). The most frequent viruses detected in pediatric outpatients and inpatients were HRV and RSV, respectively, while influenza viruses were the first in line among both adult outpatients and inpatients. During the study period, a PIV-3 outbreak (n= 96) have emerged between December 2004-April 2005, and an influenza A (H1N1)pdm09 outbreak (n= 207) between November 2009-January 2010. When the seasonal distribution was considered, the isolation rates of 1705 respiratory viruses in winter, spring, summer and autumn were 44.4%, 27%, 8.3% and 20.3%, respectively. RSV was most frequently detected from December to March, influenza viruses from November to March, HRV from December to June, and mixed infections from January to February. In conclusion, the data of our study obtained in about 12-year period indicated that the prevalence of respiratory viruses in acute respiratory infections is 33.4%, and they typically active during the months of winter and early spring in our region.