The overall objective of this study was to measure the air pollutant concentrations inside and outside of diesel truck cabs under conditions of extended idling at a truck stop. The measurements were conducted under different modes of engine and air conditioner operation at different times of day and night. One-hour average concentrations of fine particulate matter, nitrogen oxides, and carbon monoxide were measured. All trucks showed some level of self-contamination of in-cab air quality from engine emissions during idling. Some trucks showed significantly higher in-cab concentrations than outside concentrations, indicating engine compartment leaks into the cab. Other trucks showed in-cab concentrations similar to or even lower than outside concentrations but showed higher outside and in-cab concentrations during engine idling than when the engine was turned off. In these cases, truck emissions raised the outside concentrations, which then migrated into the truck cab. Carbon monoxide concentrations measured in the cab were insignificant compared with relevant air quality standards, but fine particulate matter and nitrogen dioxide concentrations were higher than some U.S. Environmental Protection Agency ambient air quality standards. None of the measurements was higher than Occupational Safety and Health Administration or National Institute for Occupational Safety and Health air quality standards.