The bispectral index (BIS) was used to examine the hypnotic efficacy of propofol in 25 smokers (20 cigarettes/day for 2 years) and 24 matched non-smokers (same gender, age, height, weight). BIS was recorded at baseline, at four incremental effect-site concentrations of propofol and at loss of consciousness. Compared with non-smokers, smokers were found to have higher BIS values at baseline (mean (SD)) (97 (1) vs 98 (1)), at 0.7 microg x ml(-1) (95 (3) vs 97 (1)) and at 1.1 microg x ml(-1) (89 (6) vs 94 (4)), p = 0.0099, and they lost consciousness at higher propofol concentrations (2.0 (0.4) vs 2.4 (0.8) microg x ml(-1)), p = 0.03, and at lower BIS values (66 (10) vs 60 (10)), p = 0.04. The hypnotic efficacy of propofol is reduced when used at low effect-site concentrations in smokers. This phenomenon may have some impact on the management of smokers undergoing sedation using target controlled infusion systems.