In amnesiacs, stimuli that at first can be recalled are usually forgotten within 1 min, but the conditions required for this severe forgetting have remained unknown. To examine this, six patients with amnesia due to head injury or stroke and six normal controls heard lists of words (Experiment 1) and stories (Experiment 2). These stimuli were to be recalled immediately or after an extended test delay (10 min in Experiment 1; 1 h in Experiment 2). Although severe forgetting occurred in the amnesiacs following activity-filled delays, much less forgetting occurred in four of these patients after delays spent in a dark, quiet room. This was true even when the patients appeared to sleep during the delays. The results show, in a novel manner, that one deficit underlying their amnesias is vulnerability to retroactive interference.