Qualified quality control is a necessary part of modern haematology: we cannot deem laboratory results reliable until effective quality checking is included. Quality assessment includes external (national, regional) and internal (intra-laboratory) control. We suggest to found a Czech national external quality control agency for haematology based on principles which were declared by the World Health Organization. Internal control includes correct identification of samples, analytical control (standardization of test, evaluation of accuracy and precision) and biological quality control (history of patient's characteristics, relation to other laboratory results in clinical context). The deviation index, which is expressed either as a number or verbally 'excellent', 'good', 'satisfactory', or 'unsatisfactory', can supply rapid information on the quality of results in the haematological laboratory.