Application of Total Quality Management concepts to laboratory testing requires that the total process, including preanalytical and postanalytical phases, be managed so as to reduce or, ideally, eliminate all defects within the process itself. Indeed a "mistake" can be defined as any defect during the entire testing process, from ordering tests to reporting results. We evaluated the frequency and types of mistakes found in the "stat" section of the Department of Laboratory Medicine of the University-Hospital of Padova by monitoring four different departments (internal medicine, nephrology, surgery, and intensive care unit) for 3 months. Among a total of 40490 analyses, we identified 189 laboratory mistakes, a relative frequency of 0.47%. The distribution of mistakes was: preanalytical 68.2%, analytical 13.3%, and postanalytical 18.5%. Most of the laboratory mistakes (74%) did not affect patients' outcome. However, in 37 patients (19%), laboratory mistakes were associated with further inappropriate investigations, thus resulting in an unjustifiable increase in costs. Moreover, in 12 patients (6.4%) laboratory mistakes were associated with inappropriate care or inappropriate modification of therapy. The promotion of quality control and continuous improvement of the total testing process, including pre- and postanalytical phases, seems to be a prerequisite for an effective laboratory service.