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One of the most elusive developmental disabilities to diagnose during infancy is cognitive delay. Although infant intelligence tests have been identified repeatedly as poor predictors of future IQ, these predictions have typically been based on interage correlations. This retrospective study examined the sensitivity, specificity, and positive and negative predictive values of the Mental Scale of the Bayley Scales of Infant Development in detecting cognitive delay (IQ< 85) among a sample of 196 high-risk, mostly low birth weight, premature infants who were assessed initially at 4 months' corrected age and followed longitudinally to between 3 and 8 years' corrected age. As hypothesized, the Bayley Mental Scale was better at correctly identifying children with future normal IQs (specificity= 91.9%) than it was at identifying infants who would later show cognitive delays (sensitivity= 38.9%). The positive and negative predictive values were 51.9% and 87.0% respectively. Results of this study support recent assertions that cognitive delay, particularly in the mildest forms, is difficult to detect during early infancy.