Preanalytic errors are important factors that affect laboratory test results, but they are generally underestimated in routine practice. As a part of a quality management program, preanalytic errors were observed in the Central Laboratory Clinical Biochemistry Unit at Ankara University Ibni Sina Hospital. Closed monitoring of preanalytic errors for hospitalized patients during a randomly selected 1-month period was performed separately for 2 different working hours [morning (08:00–16:30) and night shifts (16:30–08:00) plus weekends]. The most common preanalytic error was incorrect patient information being entered into the hospital information system (16.5% and 23.5% for the morning and night shift, respectively). The second most common preanalytic error was discordance between test requests on the forms and what was entered into the hospital information system (10.1% and 7.1% for the morning and night shift, respectively) Inadequate volume of blood (3.2% and 2.9% for the morning and night shift, respectively) was the next most common preanalytic error. The statistical differences between morning and night shifts were significant (P<0.001) for hospital information system patient errors, discordance of test requests, and hemolysis or coagulation of blood for complete blood count. Hospital information system corrections were taken into consideration as a first step in quality management of our hospital laboratory.