Human polyomaviruses, namely BK (BKV) and JC (JCV) viruses are small DNA viruses that cause latent infections worldwide. Primary infections are usually acquired in the early periods of life and are generally asymptomatic. However BKV/JCV infections may cause severe clinical conditions in immunosuppressive patients such as bone marrow and solid organ transplantation or cancer patients. The aim of this retrospective study was to investigate the presence of BKV and JCV nucleic acids by real-time polymerase chain reaction (RT-PCR) in the clinical samples of patients with high risk. A total of 268 (62 blood, 206 urine) samples obtained from 115 immunocompromised patients hospitalized in Gazi University Hospital between July 2007 to January 2009, were included to the study. Viral nucleic acids were extracted from the samples with High Pure PCR Template Preparation Kit (Roche, Germany). By using amplification mix (TIB Molbiol GmbH, Germany) that included primers targeting 174 (JCV) and 219 (BKV) base pair fragments of the small t antigen, and hybridization probes (Roche, Germany), nucleic acids were amplified with LightCycler (Roche Applied Science, Germany) system. As a result, total polyomavirus DNA positivity rate was found as 33.2% (89/268). When BKV and JCV DNA positivities were evaluated according to the samples, 25.2% (53/206) of urine samples yielded positive results for BKV, 14.5% (30/206) for JCV and 2.4% (5/206) for both BKV and JCV. Only one of the blood samples (1/62; 1.6%) were found positive by means of BKV DNA, while none of the blood samples were positive for JCV DNA. The distribution of BKV and JCV DNA positivity rates according to the inpatient clinics were as follows, respectively; 24.3% and 9.5% for pediatric nephrology, 9.6% and 8.2% for renal transplantation unit, 13.5% and 18.9% for adult nephrology, 30.8% and 15.4% for bone marrow transplantation unit, 22.9% and 8.6% for pediatric clinics. In samples from pediatric hematology patients, BKV positivity was 36.4% (4/11), while there were no JCV positivity. However, in hematology patients, while JCV was positive in one of the three samples, no BKV positivity was detected.BKV was seen in three of six samples obtained from patients in the intensive care unit. JCV was positive in both of the two samples obtained from patients in pediatric endocrinology. The only patient that had BKV DNA in blood sample was a renal transplant patient. BKV + JCV DNAs were positive together in only five (1.9%) of the urine samples. In 24% (22/89) of the samples, BKV DNA was found ≥ 10⁷ copies/ml, in 2.2% (2/89) JCV DNA was ≥ 10⁷ copies/ml, whereas in 2.2% (2/89) of samples both BKV and JCV DNA was ≥ 10⁷ copies/ml. All of those samples with high DNA levels were urine. The data of this study led to the establishment of a collaborative algorithm between the laboratory and clinics in our hospital for the diagnosis and follow-up of the patients in terms of BKV/JCV infections. In conclusion, since BKV/JCV reactivations and infections are crutial in immunosuppressive patients, especially medical centers specialized in bone marrow and renal transplantation, diagnostic and monitoring procedures related to those infections should be programmed.