This position paper advocates the necessity for promoting a Human Factors Engineering (HFE) approach to new clinical applications in the medical informatics domain. We first describe the negative consequences of NOT using HFE methods for such sensitive applications as medication Computerized Physicians Order Entry systems. We then describe rapidly the HF technology and how it would apply to the healthcare domain. The specific and complex problem of the necessary re-engineering of existing applications is then addressed. We conclude on the mandatory cooperative characteristic of any practical HFE project for designing or re-engineering safety-sensitive clinical applications.