Reliable and valid assessment of aberrant behaviors is essential in empirically verifying prevention and intervention for individuals with intellectual or developmental disabilities (IDD). Few instruments exist which assess behavior problems in infants. The current longitudinal study examined the performance of three behavior-rating scales for individuals with IDD that have been proven psychometrically sound in older populations: the "Aberrant Behavior Checklist (ABC)", the "Behavior Problems Inventory (BPI-01)," and the "Repetitive Behavior Scale-- Revised (RBS-R)." Data were analyzed for 180 between six and 36 months old children at risk for IDD. Internal consistency (Cronbach's alpha) across the subscales of the three instruments was variable. Test-retest reliability of the three BPI-01 subscales ranged from 0.68 to 0.77 for frequency ratings and from 0.65 to 0.80 for severity ratings (intraclass correlation coefficients). Using a multitrait-multimethod matrix approach high levels of convergent and discriminant validity across the three instruments was found. As anticipated, there was considerable overlap in the information produced by the three instruments; however, each behavior-rating instrument also contributed unique information. Our findings support using all three scales in conjunction if possible. (Contains 2 figures and 6 tables.)