Reliability and concurrent validity of the Behavior Problems Inventory (BPI-01; Rojahn et al., 2001) was examined in a sample of 130 community residing adults with mild to profound intellectual disabilities with high rates of behavior problems and concurrent mental health problems. The BPI-01 and the Inventory for Client and Agency Planning (ICAP; Bruininks et al., 1986) were administered twice within a mean time interval of 7.8 weeks by 20 trained and experienced staff members. All three BPI-01 subscales had high inter-rater agreement (Self-Injurious Behavior [SIB]: mean ICC = 0.84; Stereotyped Behavior: mean ICC = 0.75; Aggressive/Destructive Behavior: mean ICC = 0.82), and stable test-retest reliability (SIB, mean ICC = 0.91; mean Stereotyped Behavior, mean ICC = 0.89, and Aggressive/Destructive Behavior, mean ICC = 0.88); internal consistency ranged from poor (SIB: alpha = 0.61) to excellent (Stereotyped Behavior, alpha = 0.90). Using the ICAP as criterion measure, the BPI-01 showed robust convergent validity. Solid relationships between BPI-01 subscales and corresponding ICAP subscales corroborated the concurrent validity of the BPI-01. (Contains 7 tables.)