This "Monograph" reports theoretically relevant behavioral, biological, and self-report assessments of a sample of 14-17-year-olds who had been classified into one of four temperamental groups at 4 months of age. The infant temperamental categories were based on observed behavior to a battery of unfamiliar stimuli. The infants classified as high reactive (20% of the sample) displayed vigorous motor activity and frequent crying. Those classified as low reactives (40%) displayed minimal motor activity and crying. About 25% of the infants, called distressed, showed minimal motor activity but cried frequently, and 10%, characterized by vigorous motoricity but little crying, were called aroused. Evaluations of these children at 14 and 21 months, and 4, 7, and 11 years revealed that the high reactives were most likely to be avoidant of unfamiliar events at the early ages and emotionally subdued, cautious, and wary of new situations at the later ages. By contrast, the low reactives were the least avoidant of unfamiliarity in the second year and most emotionally spontaneous and sociable at the later ages. The participants in the other two groups displayed intermediate profiles. At age 11 years, the high reactives were more likely than the low reactives to display right hemisphere activation in the EEG, a larger evoked potential from the inferior colliculus, larger event related waveforms to discrepant scenes, and greater sympathetic tone in the cardiovascular system. The current evaluation revealed that, at 14-17 years of age, more high than low reactives were likely to be subdued in unfamiliar situations, to report a dour mood and anxiety over the future, to be more religious, to display sympathetic tone in the cardiovascular system, to combine a fast latency with a large magnitude of the evoked potential from the inferior colliculus, and to show shallower habituation of the event-related potential to discrepant visual events. As at earlier ages, there were low correlations among the biological measures. However, there was stronger dissociation between behavior and biology at age 15 than at earlier ages. Finally, infant temperamental category at 4 months was not a less powerful predictor of behavior at age 15 than the combination of temperament and fearful behavior in the second year. (Contains 6 tables and 3 figures.)