This study investigated the pattern of acute cognitive function of an ecologically valid sample of 58 consecutively admitted adults completing a 24 day residential/day substance use disorder treatment program at a Veterans Affairs Medical Center and its change over the 24 days of monitored abstinence. Participants were assessed at entry with the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) Form A and at treatment conclusion with the RBANS Form B and with the Similarities subtest of the Wechsler Adult Intelligence Scale-III (WAIS-III) and the Trail Making A and B tests at both time points. There were significant (p < 0.002) improvements in the RBANS Immediate Memory, Attention, and Total Index scores over the course of treatment and on the Similarities subtest and the Trail Making A and B tests with effect sizes ranging between 0.37 and 0.56. There was greater cognitive performance increase for participants in the lower versus upper quantiles at treatment entry on all cognitive measures.