Guidelines for the multiple sleep latency test (MSLT) were published in 1986 and updated in 2005. Individual interpretation of instructions may lead to variability in conducting and reporting the test with implications for diagnosis.To assess variability in conducting MSLT among sleep laboratories in Europe.A questionnaire based on the clinical MSLT guidelines was posted or emailed to 283 sleep centres in Europe.Ninety adult laboratories performing MSLT returned the questionnaire. Indications for MSLT were: narcolepsy, excessive daytime somnolence, driving issues and assessment of wakefulness. Routinely, only 17% of laboratories performed urinary drug screening and 68.2% asked patients to complete sleep diaries. Overnight polysomnography before MSLT was ran in 87.5% of laboratories. There was variation in setup and instructions among countries. Sleep onset was scored as one epoch of any stage sleep by 65.9% of labs. REM latency was calculated and reported according to the 2005 guidelines in 73.7% of cases. Abnormal daytime sleepiness was considered to be a sleep latency⩽5min in 33% laboratories.There is marked variability in the practice and interpretation of the MSLT in Europe.