A 17-month sampling campaign was initiated to investigate the potential for vegetated highway shoulders to retain suspended solids, metals, and total petroleum hydrocarbons (TPH). A site along SR 8 in Western Washington was selected and three full-scale test plots constructed for evaluation of contaminant retention capability. The data indicated that TPH and suspended solids were effectively removed. Metal concentration reduction was also effective when consideration was given to inadvertent pretreatment afforded by the highway runoff collection system. Consequently, the vegetated highway shoulder, located along hundreds of miles of highway can provide a cost effective means of contaminant retention.