Use of drugs and alcohol by 43 predominantly male outpatients who had severe mental illness and a comorbid substance use disorder were assessed weekly through the ratings of experienced dual disorder case managers and through blinded research urine toxicology tests. The percentage of weeks in which drugs or alcohol were used was calculated on the basis of one or both assessments. The case managers often missed drug use over the weekends, which was detected by the urine toxicology tests. Agreement between the two methods varied widely, even when the ratings were made by highly experienced case managers. These findings have implications for monitoring patients with dual diagnoses and provide insight into the accuracy of case manager ratings.