A conversion to light-duty diesel vehicles is considered one pathway for reducing transportation greenhouse gas emissions in the United States. However, expanding diesel's market share could have significant public health consequences. This paper evaluates the environmental tradeoffs of diesel passenger vehicles, and suggests that a rapid conversion to light-duty diesel vehicles could yield carbon emission savings within the decade of up to 4 percent versus the base case. While significant reductions in criteria emissions appears technologically possible, it will likely require that diesels incorporate two exhaust control systems and utilize reformulated diesel fuel or alternative fuels. Finally, we suggest that diesel manufacturers must address other evolving public health concerns, including emissions of ultrafine particles and toxics, before expanding the market share of the diesel powerplant.