Background Infants with very low birth weight (VLBW) are at increased risk for motor deficits, which may be reduced by early intervention programs. For detection of motor deficits and to monitor intervention, different assessment tools are available. It is important to choose tools that are sensitive to evaluate the efficacy of intervention on motor outcome. Objective The purpose of this study was to compare the Alberta Infant Motor Scale (AIMS) and the Psychomotor Developmental Index (PDI) of the Bayley Scales of Infant Development–Dutch Second Edition (BSID-II-NL) in their ability to evaluate effects of an early intervention, provided by pediatric physical therapists, on motor development in infants with VLBW at 12 months corrected age (CA). Design This was a secondary study in which data collected from a randomized controlled trial (RCT) were used. Methods At 12 months CA, 116 of 176 infants with VLBW participating in an RCT on the effect of the Infant Behavioral Assessment and Intervention Program were assessed with both the AIMS and the PDI. Intervention effects on the AIMS and PDI were compared. Results Corrected for baseline differences, significant intervention effects were found for AIMS and PDI scores. The highest effect size was for the AIMS subscale sit. A significant reduction of abnormal motor development in the intervention group was found only with the AIMS. Limitations No Dutch norms are available for the AIMS. Conclusions The responsiveness of the AIMS to detect intervention effects was better than that of the PDI. Therefore, caution is recommended in monitoring infants with VLBW only with the PDI, and the use of both the AIMS and the Bayley Scales of Infant Development is advised when evaluating intervention effects on motor development at 12 months CA.