Nosocomial infections constitute an important health problem with morbidity, and high mortality, prolongation of hospital stay, and increased costs of direct patient care. This study was conducted in Hacettepe University Hospital in order to determine the cost of nosocomial infections and length of hospitalization by matching infected patients with uninfected controls. Data collected from 102 individuals with nosocomial infection (group A) and 102 controls without nosocomial infection (group B) were recorded by using a computer program (dbase IV). Urinary tract infections, surgical wound infections and bacteremias were the most common nosocomial infections. Patients were generally located in Internal Medicine, Neurosurgery, General Surgery Clinics. The mean total length of stay was 35.1 and 14.8 days for group A and group B, respectively (p<0.001). The average hospital cost was US $2280 for group A, and US $698 for group B (p<0.001). Mortality rates were 19.6% for patients with nosocomial infections and 2.9% for uninfected controls (p<0.0001). We conclude that the high economic expense which nosocomial infection represents justifies measures to control this entity.