The "z-drugs" zopiclone, zolpidem, eszopiclone, and zaleplon were introduced in the 1980s for the treatment of insomnia, as it was observed that the side effect profile associated with these medications were more benign than those related to the benzodiazepines. This meta-analysis set out to ascertain which domains of cognitive function, if any, were affected by the ingestion of these medications. A total of 20 studies met the study inclusion criteria. Results revealed medium effect sizes for zopiclone and zolpidem on measures of verbal memory. An additional medium effect size was observed for zolpidem on attention. Finally, smaller effect sizes were observed for zolpidem speed of processing and for zopiclone on working memory. It is clear from these data that the use of a single dose of the z-drugs in healthy adults as measured in the morning following the exposure does produce a specific rather than a generalized negative effect on cognitive function. However, there were only enough studies to evaluate the individual cognitive effects of the zolpidem and zopiclone medications; the specific effects of zaleplon and eszopiclone cannot be ascertained because only one study met the inclusion and exclusion criteria for the review.