The centralized preparation of cytotoxic drugs is supposed to help preventing medication errors. Nevertheless, the residual risk of preparation errors has been demonstrated. Above all, prevention is based on the quality assurance policy. Following a previous work, this study valuates the effectiveness of a quality assurance policy and the impact of corrective actions. The study compares the rates of preparation errors observed before and after the corrective actions. The risk factors have also been studied. The study included 84,017 consecutive preparations. The results showed a significant reduction of overall (0.2% versus 0.4%) and major (0.1% versus 0.2%) errors between the two periods. This period impact has been validated by a multivariate analysis (OR were respectively 0.49 and 0.6, p < 10(-3)). The daily workload (> 60 preparations) appeared to be the most important risk factor (OR were respectively 2.4 and 3.2, p < or = 10(-3)). These results demonstrated the effectiveness of simple corrective actions. The follow-up on preparation errors appeared to be useful to raise technicians' awareness and to motivate them. An epidemiological approach allows to identify risk factors, and to definite relevant corrective actions.