The multiple sleep latency test provides an objective measure of a patient's daytime sleepiness. Sixteen tests were performed at Groote Schuur Hospital in 1987 and 1988 according to a fixed protocol. In 8 patients the test was definitely abnormal (mean sleep latency less than 5 minutes) with 3 subjects diagnosed as having narcolepsy, 1 sleep apnoea syndrome, 1 idiopathic central nervous system hypersomnolence, 2 environment-related hypersomnolence and 1 psychophysiological hypersomnolence. In 2 patients the test results fell in the equivocal range (mean sleep latency 5-10 minutes), while in 5 the test revealed no evidence for a disorder of excessive sleep (mean sleep latency greater than 10 minutes). The test was uninterpretable for technical reasons in only 1 patient. In conclusion, the test--when performed in a standardised manner--is extremely helpful in the elucidation of possible disorders of excessive sleep.