Key properties of polyimide-coated optical fibers, unaged and exposed to various harsh environments, were investigated. The main intent was to model extreme conditions that can be encountered in medical applications of the fibers. A fiber designed by OFS showed good strength and was able to withstand exposure to extreme heat and humidity, multiple autoclave cycles, extended water soak and immersion in organic solvents. Similar fibers offered by other suppliers displayed shortcomings in some of the tested properties.