Runner's knee is pain and inflammation in the cartiledge under the kneecap. Usually this results from a muscle imbalance around the kneecap, weak quads or overpronation. Running on sidehills or downhills will irritate this condition, moreso than uphills.
http://www.runningonline.com/zine/Injuries/91.sht
"Prevention: Avoid Downhills"
http://www.medinfosource.com/expert/exp3031201a.html
"Chondromalacia patella can oftentimes be treated by decreasing or eliminating the exercise that is causing the problem (such as running down hills)"