The CAT/CLAMS, a pediatric tool for cognitive assessment in infants and toddlers, correlates well with the Bayley Scales of Infant Development-Mental Scale. The Bayley Scales of Infant Development, 2nd Ed. (Bayley II) was developed in 1993. Objective: This study was completed to determine how well the CAT/CLAMS correlates with the Bayley II and its utility in identifying mild mental retardation (MR). Methods: Subjects included 68 infants and toddlers (age 14-48 months) referred for suspected developmental delays. Each subject was administered the CAT/CLAMS by a pediatrician and the Bayley II by a psychologist. Each examiner was unaware of the results of testing with the other instrument. Results from each instrument were expressed as developmental quotients. Results: A highly significant correlation (r=0.89, p<0.0001) between the two instruments was found. The relationship between the CAT/CLAMS and the Bayley II was not influenced by race, sex, or gestational age. The CAT/CLAMS was sensitive (90%) and specific (78%) for detecting MR (Bayley II<70). The CAT/CLAMS was even more sensitive (100%) and specific (96%) in detecting severe MR (Bayley II<50). When compared with the final diagnosis of MR from the psychological assessment, the physician utilizing the CAT/CLAMS formulated an initial clinical impression of MR that was sensitive (95%) and specific (84%). Conclusions: The CAT/CLAMS correlates well with the Bayley II and can be used as an instrument for detecting and quantifying cognitive delay. It permits the physician to formulate an accurate clinical impression of probable MR and to make referrals for definitive diagnosis. It is useful in detecting severe MR, making it an important clinical tool to trigger further etiologic investigation and to plan interventions.