The predictive relation between neonatal reactivity and 12-, 18-, and 24-month temperament was assessed for 22 appropriate-for-gestational-age (AGA)/small-for-gestational-age (SGA) twin pairs. Neonatal reactivity included ratings of visual and auditory orienting responses to a bull's-eye, rattle, bell, voice, and face plus voice combined, as well as alertness. Infant temperament ratings were made of emotional tone, activity, attentiveness, and social orientation to staff during ageappropriate activities in the laboratory. There were no group differences in the neonatal or infant ratings. For AGA twins, a pattern of predictive correlations was obtained between neonatal reactivity and temperament at each age. A similar predictive pattern was not observed for the SGA twins. Group differences in rates of maturation related to stresses associated with intrauterine growth retardation influenced the significance of the neonatal variables for later temperament development.