Abstract Informant reports of psychiatric patient behavior are collected routinely during intake interviews and to monitor therapeutic interventions. We investigated agreement between informant and adult psychiatric patient reports of patient behaviors (N=110). Behaviors were about substance use, physical and mental health, employment, illegal behavior, and recreational and social activities. Informant agreement with patients regarding whether or not a particular behavior occurred recently and also behavior frequency ratings were moderate overall (M r=.47 for frequency) and highly variable (rs=−.09 to .80). No informant, patient, or relationship characteristics were found consistently to moderate agreement. Thus, informants can report very accurately about the presence and frequency of some, but not all, patient behaviors. Low patient-informant agreement may be due to informants having relatively less detailed knowledge of patient behavior over time or patients not reporting accurately about certain behaviors.