The recurrence of dengue fever cases in Northern Taiwan has further amplified the linkage between environment and health events. Taking seven major cities in Taipei prefecture as a case study, this paper intends to set up a signal system for assessing environmental sanitation and carry it out in 1996. Major findings indicated that: (1) environmental quality is in rather an unstable status in the study area; (2) public sanitation has not yet become part of their daily life among residents; and (3) dengue fever, often accompanied with amplifiers such as flood, will appear again in these cities. In addition to the design and implement of the signal system, an average score method has been developed in this study to measure the sanitation in an area as an objective standard?