Objectives: The aims of this study were to determine the usage patterns and the cost of antibiotics, along with the evaluation of the effects of infectious diseases (ID) specialists on appropriate antimicrobial use in hospitalized patients. Materials and methods: A one-day, cross-sectional study was conducted in a major tertiary hospital and data on the use of antibiotics were collected by using a standard form. The appropriateness of the antibiotic usage was evaluated using the Council for Appropriate and Rational Antibiotic Therapy (CARAT) criteria; and the consumption and daily cost of antibiotics were determined. Results: On the study day, antibiotics were prescribed in 199 (35.6%) of 553 hospitalized patients, in 109 (32.9%) on the surgical and 90 patients (40.5%) on the medical wards. The total empirical antibiotic use was more frequent (49.7%) than prophylactic (29.1%) and culture-based therapy (21.2%). In 44 patients (22.1%) the antibiotics were used inappropriately; any of these antibiotics needed the approval of ID specialist. The inappropriate usage was more common in prophylactic therapy (46.5%) than empirical (16.1%) and specific antibiotic administration (2.3%). ID consultation rates were significantly higher in the appropriate antibiotic administrations (69.6%) than in the inappropriate group [(6.8%, p<0.0001), odds ratio (OR) 10.2, confidence intervals (CI) =3.0&ndash;3.7]. The total one-day cost of antibiotic therapy in our hospital was US $3350.6, and the total daily cost for hospital infection was $2137.1. The mean daily cost per patient was $2.1 for prophylaxis, $10.7 for community-acquired infections and $54.7 for hospital infections (p<0.001, OR 9.8, CI 4.7&ndash;20.7). Conclusion: This study showed that antibiotic prescription rates are high, surgical prophylaxis is still a major problem in our hospital, ID approval is effective for appropriate use of antibiotics, and the antibiotic cost of hospital infections is an important part of extra costs.