In real-world settings, execution of retrieved intentions must often be briefly delayed until an ongoing activity is completed (delayed-execute prospective memory tasks). Further, in demanding work settings, the ongoing activity itself can be interrupted. Experiment 1 examined the effects of the delay length (5, 15, 40 s), the presence of an interruption within that delay, and the length of the interruption on prospective memory performance. Delay length did not significantly affect performance, but interruptions produced significant decrements in performance relative to a delay alone. The length of the interruptions (10 vs. 20 s) did not significantly affect performance. Experiment 2 replicated the negative effects of interruptions, and found that these effects could be overcome with a simple external mnemonic. We suggest that in demanding work environments where interruptions are likely, external cues are advisable, especially where prospective memory failures have critical consequences. Copyright © 2004 John Wiley & Sons, Ltd.