Background and objectives: Drug-dependent people exhibit biases when evaluating discrete emotional facial expressions. Little is known about how drug abusers process multiple expressions presented simultaneously. The present study investigated the number perception of schematic emotional expressions by abstinent heroin abusers. Methods: Eighty-four heroin abstainers with varied lengths of abstinence (short-term, mid-term, and long-term) and twenty healthy controls were examined. A method of limits was deployed to obtain estimates (points of subjective equality) of perceived numbers of schematic faces (expressing positive, neutral, or negative emotion). Results: Major results include the following: 1) heroin-abstinent participants showed significantly lower points of subjective equality for negative and neutral faces, but not for positive faces, compared to control participants: 2) heroin-abstinent participants showed lower points of subjective equality for negative faces and higher ones for positive faces when compared to neutral faces, while no such differences were found in control participants. Conclusion: Heroin abusers demonstrate an exaggerated perception of number when exposed to negative expressions, even after a period of abstinence as long as 10 months. In addition, the current results could also reflect an underestimated perception of number during exposure to positive expressions and a heightened baseline for neutral expressions, or the attribution of negative valence to neutral expressions by heroin abusers. (C) 2011 Elsevier Ltd. All rights reserved.