The Questions About Behavioral Function (QABF), a 25-item rating scale, was developed to identify the function(s) of maladaptive behavior in individuals with developmental disabilities. The authors adapted it for use with individuals with serious mental illness who engage in maladaptive behavior and assessed the psychometric characteristics of the new scale (Questions About Behavioral Function in Mental Illness; QABF-MI) in a sample of 135 adults with serious mental illness from three inpatient psychiatric hospitals. Staff most familiar with each person rated each item on a 5-point Likert-type rating scale, and the ratings were subjected to a number of psychometric analyses. The results of factor analyses provided a conceptually meaningful five-factor solution: physical discomfort, social attention, tangible reinforcement, escape, and nonsocial reinforcement. Congruence between the five factors derived with the QABF-MI and the corresponding factors in the original QABF was perfect. The results indicated that the QABF-MI has robust psychometric properties and may be useful as a screening tool for determining the nature of the variables that maintain maladaptive behavior exhibited by individuals with serious mental illness. (Contains 2 tables.)