As more people are connected to the Internet, researchers have been growingly concerned with Internet addiction and the psychological attributes connected to it. The aim of this study is to examine the relationship between Internet addiction and self-esteem. The sample included 1399 Portuguese and Brazilian Internet users, from 14 to 83 years old, who responded to the Internet Addiction Test (IAT) (Young, K. (1998b) Caught in the Net: How to recognize the signs of Internet addiction and a winning strategy for recovery, John Wiley & Sons, Inc., New York) and the Rosenberg Self-Esteem Scale (RSES) (Rosenberg, M. (1989) Society and the adolescent self-image, Wesleyan University Press, Middle Town). A factor analysis of the IAT was conducted, revealing three factors: withdrawal & concealment, social & personal consequences, and excessive use. Using a Pearson correlation, we found a negative correlation between Internet addiction and self-esteem. Linear regression indicated that low self-esteem explained 11% of Internet addiction, and that negative feelings caused by Internet addiction (withdrawal & concealment) explained 13% of self-esteem. In the analysis of the IAT, we found that the groups exhibiting elevated levels of Internet addiction included men, Brazilians, and youths (14-25 years old). RESEARCH HIGHLIGHTS This paper reports on a cross-cultural study in Portugal and Brazil with a large sample of Internet users. Results indicate that: There is a negative correlation between Internet addiction and self-esteem, which suggests that subjects with a low self-esteem present higher levels of Internet addiction. It is not Internet addiction itself that is damaging to self-esteem, but the negative emotions (i.e. sadness, irritability, distress, disguise and deceptive behavior) experienced by users suffering from Internet addiction, as demonstrated by the prediction pathways analyzed. Levels of Internet addiction appear to decrease with age, however, every age group has addicted users, disclosing that it is a concern in every generation and not just a problem associated with the youth. This study suggests that the link between low self-esteem and Internet addiction underlies the psychological character of a possible new disorder with real symptoms and consequences.