Introduction: Error in blood sampling is of one the commonest causes of laboratory sample rejection and poses a great challenge particularly amongst oncology patients due to difficult venous access. This study aims to identify the main causes of blood sample rejection in the haematology and chemical pathology (CP) laboratories of an oncology institute. Method: All blood samples received and rejected in the CP and haematology laboratory from 2017 to 2019 were obtained from the laboratory information system (LIS) and sample rejection logbook. The rejection cause for each of the rejected samples was recorded and analysed. Results: Out of the total 39 495 blood samples received, 244 (0.6%) were rejected. The rejection rate in the CP was higher compared with that in the haematology laboratory (51.2% vs. 48.8%). The most frequent cause of rejection was haemolysis (49.6%), clotted sample (32.8%), and insufficient sample volume (6.1%). Conclusion: Haemolysis, clotted blood and insufficient sample were the main causes of sample rejection in our oncology centre. Effective and multidisciplinary targeted interventions to reduce blood sampling error are important to improve pre-analytical handling of blood samples from oncology patients.