Multiple Sleep Latency Test (MSLT) is of importance in diagnosis of hypersomnia and narcolepsy. The test is, however, rather time-consuming and expensive. We wanted to study the diagnostic value of the test when repeated four instead of frequently used five times.91 MSLTs with five repeated sleep latency tests were performed in our laboratory during 1994-95. 12 patients had narcolepsia, 60 hypersomnia, 11 OSAS and 8 other sleeping disturbance as working diagnosis. Mean age was 44 years, range 16-64. MSLT was performed using standard recommendations (EEG from central and occipital areas, EOG, EMG, ECG, body and respiratory movements). 5 tests (at 0900, 1045, 1230, 1415 and 1600 h) were performed. Latency to the first 20 s epoch of S1 sleep (S1 latency), 3 consecutive epochs of S1 (sleep latency, SL) and the possible REM-onset during first 10 min of sleep were determined. In statistical analysis the results were compared with and without the last measurement.The mean SL in first 4 recordings was 2.8 min and in 5 recordings it was 2.6 in narcolepsia, 9.510.0 in OSAS, 10.310.3 in hypersomnia and10.810.5 in other sleeping disturbances, respectively. As there was no significant difference in the realibility of MSLT with 4 or 5 measures in any patient group and the results did not affect to the final diagnosis, we can recommend the use of 4 tests for clinical purpose.