To evaluate whether conceptional or chronologic age should be used to determine scores in developmental follow-up studies, a study was made of 236 normal and 66 neurologically abnormal infants who were similar with respect to conceptional age but different with respect to degree of prematurity. Assessments of possible differences in cognitive and motor performance were made with the Bayley Scales of Infant Development and a neurological examination, which were administered to all babies at 9 and 18 months conceptional age. Infants were divided into four age groups to reflect differing degrees of prematurity: gestational age less than or equal to 32 weeks, 33 to 34 weeks, 35 to 36 weeks, and equal to or greater than 37 weeks. Cognitive and motor raw scores did not differ significantly across the prematurity groups for either neurologically normal or abnormal infants at 9 and 18 months conceptional age, despite differing chronologic ages. Compromised developmental function was detected as indicated by differences between neurologically normal and abnormal babies, especially in the more premature groups. It is concluded that the use of conceptional age as opposed to chronologic age at 9 and 18 months adjusts for "catch-up" due to prematurity, but does not appear to "cover-up" dysfunction. (Author/RH)