The multiple sleep latency test and the maintenance of wakefulness test were administered on the same day to 258 consecutive patients whose clinical presentation required evaluation for excessive sleepiness. While the MSLT is the standard test for assessing excessive daytime sleepiness, the MWT may have some clinical advantage over the MSLT when the assessment of daytime alertness is the primary goal. To explore further the relationship between alertness and sleepiness, we have conducted a thorough analysis of the similarities, differences, and correlations between MWT and MSLT. The results of this study show that the coefficient of correlation between MSLT and MWT (r = 0.41), although statistically significant, accounts for less than 17 percent of the variability between the two tests. Factor analysis suggests that two factors, alertness and sleepiness, account for 91 percent of all variance. Our data demonstrate that patients with diagnosable disorders of excessive somnolence may be discordant on the two tests (eg, having low sleep latency on MSLT but high sleep latency on MWT). Specifically, we found that some patients with abnormally low MSLT scores were able to stay awake when asked to do so on the MWT, and conversely, some patients who failed to stay awake when asked to do so on the MWT were unable to fall asleep quickly on the MSLT. We conclude that the MWT and MSLT measure different abilities and that the MWT may be a useful adjuvant daytime test in many clinical situations.