Appreciation of the importance of screening for cognitive impairment among substance abusing populations has increased in recent years. In this article, demographic effects on the Trail Making Test (TMT), a test often used for screening for cognitive impairment, are examined in a sample of patients in drug abuse treatment programs. A sample of 5619 males and 2902 females was drawn from electronic files of data from the Drug Abuse Treatment Outcome Study (DATOS). The DATOS was a naturalistic cohort study that collected data from 1991 to 1993 in 96 programs in 11 cities in the US. Data were analyzed to determine the effects of demographic variables on the two parts of the TMT in this large sample of patients. Consistent with previous research, demographic variables such as age, gender, education level, and ethnicity were statistically significantly related to both TMT Parts A and B. More importantly, however, the percentage of variance accounted for was quite small. These results suggest that, while clearly present, demographic effects on the TMT are weak.