This paper is one of a series dealing with sleep arousals accompanied by anxiety. It contains a useful classification of such phenomena as Stage 4 night terror, REM nightmare, Stage 2 anxious arousal, and hypnagogic nightmare. Full-blown Stage 4 night terrors are described as fight-flight episodes with utterances, sleepwalking, and hallucinated or delusional mental content associated with terror. There are intense autonomic changes, including marked increases in heart rate, respiratory rate and amplitude, and decreases in skin resistance. Also observed was a correlation with delta sleep on the given night, with the intensity of the episode related to the amount of the preceding delta sleep. The important issue of the relationship of night terrors to traumatic events is left uncertain. The authors suggest, however, that the night terror is not a dream, but a pathological symptom formation from NREM sleep, brought about by the ego's inability to control anxiety. This apparently differs from the REM nightmare in which there is evidence of attempts at mastery of the traumatic experience.