Health symptoms attributed to environmental odor exposure are not well understood. Cognitive factors seem to play a significant role in odor-related illness. In the present study, we investigated whether such influences are predominantly interpretational (i.e. best understood as interpretations of perceived odors), or also perceptual (i.e. affect perceptions of the characteristics of the odor). To investigate the neuronal activation behind such processes olfactory ERPs were recorded. The experiment consisted of two conditions: one where participants expected just several administrations of one odor (labelled as the "non-painful" condition), and one where they also expected, in between the odor administrations, to feel irritation in the nose (labelled as the "painful" condition). Participants received painless H(2)S stimuli during both conditions. To reinforce pain-expectancy, a CO(2) pulse was given occasionally during the "painful" condition. Crucial comparisons were made between reactions to H(2)S, under the two expectancy conditions. Detection sensitivity (reflected by amplitudes and latencies of the early N1 peak) and stimulus salience (reflected by amplitudes and latencies of the late "cognitive" positivity) were examined. Peak amplitudes were unaffected by expectancy condition. However, a significant main effect of expectancy on the N1 latency was found, which suggests that expecting a painful stimulus reduces the time to detect a harmless odor. In conclusion, expectancies seem to alter early aspects of odor perception.