Purpose. - In France, low molecular weight heparins are widely used for prophylaxis of venous thromboembolism. This practice is not based on strong evidence supporting the use of this technique, thus leading to important difficulties in defining accurate guidelines. Improvement in the prophylaxis must focus on the process leading to prescription or non-prescription. Our primary objective was to identify and describe this process. We also analyzed the causes for dysfunction and implemented corrective actions. Methods. - Basic tools and methods specific to improvement of quality were used including: flow charting for process description, causes-effect diagram for updating and analysis of dysfunction, weighted vote for decisions regarding corrective actions, and agenda and designation of leaders for the implementation of corrective actions. Results. - The theoretical process beginning with the recognition of a risk and ending with the adequate treatment or prevention of this risk was demonstrated. The most important dysfunction was the lack of epidemiological and clinical trials. Key corrective actions were the following: standardization of practices where evidence was available; if not, collaborative efforts to collect valid epidemiological data through multicenter surveys and clinical trials; systematic appraisal of the quality of published data; assessment of the risk for venous thromboembolism during hospitalization. To achieve real improvement in practice, priority was given to repeated measures of the risk for thromboembolism. The use of low molecular weight heparins has been so wide however, that it has led to some difficulties, We must draw lessons from this experience. Conclusion. - Improvement of quality in terms of healthcare is not only based on auditing the practices or modifying the defective processes, but also on our ability not to use new drugs before fully assessing their validity. (C) 1999 Elsevier, Paris.