Editors' Summary: Early this year, the Fifth Circuit, in Sierra Club v. Sigler, became the first court of appeals to interpret CEQ's "worst case" regulation for EISs. The rule requires agencies to perform a worst case analysis where significant scientific uncertainty or gaps in information exist concerning the environmental effects of a project. Because the rule is unique and only recently promulgated, it is unclear what types of projects are included within the rule or how broad the analysis must be. The Fifth Circuit found that the Corps of Engineers' deepwater port project in Galveston, Texas, which created the possibility of a major oil spill from a supertanker in a large estuary, was "precisely the type of situation" requiring such an analysis. The court's thorough analysis of the rule provides useful guidance for agencies and courts. While the rule may still prove difficult to implement, the court suggested that judicial review will not be as rigorous as review of other parts of an EIS.