To assess the usefulness of auditory steady state response index (ASSR index) in monitoring the depth of sedation with propofol.Propofol at the target plasma concentrations of 0.5, 1.0, 1.5 and 2.0 micrograms/ml respectively was administered to 30 ASA grade I-II adult patients to be operated on by computer controlled infusion technique. Two minutes after equilibration of the effect compartment's concentration electively the plasma propofol concentration, the 40 Hz ASSR was recorded immediately at every concentration. Then the infusion was discontinued. When the patients opened their eyes to verbal commands of normal intonation, the 40 Hz ASSR was recorded again. The ASSR index was calculated as the sum of the square root of the absolute difference between every two successive 0.5 ms segments of the auditory steady state response's waveform. The level of sedation was determined by the observer's assessment based on the alertness/sedation scale (OAA/S). The OAA/S score of 2 or less was considered unconscious.The amplitudes of Pa and Pb waves and the ASSR index were linearly decreasing with the increase of propofol target concentration and the deepening of level of sedation dose-dependently with the association coefficients as r = 0.780, r = 0.826, and r = 0.759 respectively, P < 0.01. The corresponding regression equations are as follows: Pa = -0.354 x target concentration +1.018, and Pb = -0.468 x target concentration +1.247, ASSR index = -2.924 x target concentration +11.78. The ASSR index reflected the amplitudes of Pa and Pb, especially the amplitude of Pa (r = 0.811 and 0.735 respectively, P < 0.01). In addition, ASSR index was correlated with the depth of patient's sedation (r = 0.815, P < 0.01). When the patients fell asleep from drowsy state, the amplitude of Pa decreased from 0.9 mu v to 0.6 mu v, and the ASSR index decreased from 10.5 to 8.6 with statistical significance (P < 0.05). The patients would not fall asleep until the ASSR index reduced to about 8.5, while reliable unconsciousness happened only when the ASSR index was lower than 5.6.ASSR index is useful in monitoring the depth of sedation with propofol. Compared with the amplitudes of Pa and Pb observed traditionally, ASSR index is more accurate and simple and convenient.