Multiple sleep latency tests (MSLT) performed on 144 patients with excessive daytime somnolence were examined for the diagnostic reliability of a short sleep latency (SL less than 5 min) and the presence of sleep-onset REM periods (SOREMPs). Based on clinical criteria, 61 patients (42%) were diagnosed as having narcolepsy. Thirty-five narcoleptic patients and five nonnarcoleptic patients exhibited a mean SL less than 5 min, yielding a sensitivity of 57% and a specificity of 94% for this criterion for pathological drowsiness. The occurrence of two or more SOREMPs was found in 52 narcoleptic patients but in only one nonnarcoleptic patient (sensitivity of 84% and specificity of 99%). Those narcoleptic patients with cataplexy demonstrated a shorter SL and more frequent SOREMPs than their noncataplectic counterparts. It was concluded that the MSLT is a highly reliable laboratory tool for the confirmation of the diagnosis of narcolepsy based on the SOREMP criterion. The criterion value for SL in pathological drowsiness may depend on laboratory conditions as well as the patient population selected.