Background and Objective: The temporal trend of adenocarcinoma incidence rates of the lung (ADL) has been reported to parallel the trend of nitrogen oxide (NO x ) emissions in the United States. This study explores the geographic pattern of ADL incidence and its relation to motor vehicle density, the major indicator of NO x emissions before 1970. Methods: ADL incidence rates by counties were retrieved from the nine sites of Surveillance, Epidemiology, and End Results (SEER) Program for the period of 1973 to 1990. Motor vehicle densities by county in 1970 for these SEER sites were estimated according to the numbers of motor vehicle registration and the size of each county. Regression analysis was done with the data of motor vehicle density by counties.