Objective: Accurate laboratory results are important in disease detection, classification, treatment, and follow-up. In this study, we aimed to evaluate records within a year for samples rejected in a biochemistry laboratory. Methods: Data of rejected samples in Ankara Children's Health and Diseases, Hematology Oncology Training and Research Hospital biochemistry laboratory between January 2015 and December 2015 were retrospectively screened from the laboratory information system. Errors were evaluated according to their type and working groups. Results: A total of 565,409 samples were sent to the biochemistry laboratory over one year. In total, 408,374 samples were sent to the central laboratory and 157,035 of them were sent to the emergency laboratory. Further, 3,309 (0.81%) samples sent to the central laboratory were rejected because of the detection of preanalytical errors, while 1,097 (0.69%) samples sent to the emergency laboratory were rejected. The more common sources of error were clotted samples and inappropriate sample volumes. Besides, more common errors were observed in hemogram and blood gas study groups. Conclusion: It is extremely important to keep the error-prone preanalytical phase that affects the quality of the results of a laboratory under control to obtain accurate and qualified results. Error proofing should be planned by taking into account the characteristics of the samples sent to the laboratory.