The 20-item Toronto Alexithymia Scale (TAS-20) purportedly measures 3 intercorrelated dimensions of the alexithymia construct: (a) difficulties identifying feelings (DIF), (b) difficulties describing feelings (DDF), and (c) externally oriented thinking (EOT). The primary purpose of this study was to evaluate the factor structure of the TAS-20 with full-information item factor analysis in 2 diverse samples: medical students (n = 219) and psychiatric (psychoactive substance dependent-abusing) inpatients (n = 204). In each group, we estimated the parameters of 1-, 2-, 3-, and 4-factor models. None of the various factor solutions resulted in clean, simple structures in either sample. In the student sample, the data were best represented (although not well) by a 3-dimensional model: DIF, DDF, and EOT. In the psychiatric sample, however, the 3-dimensional solution was quite different; DIF and DDF formed 1 "emotional awareness deficits" factor, and EOT split into 2 unrelated factors. We concluded that alexithymia, as measured by the TAS-20, is multidimensional and not well-represented by a global severity score. Our recommendations include checking the factor structure of the TAS-20 when practicable, computing subscale scores, and exercising caution in interpreting TAS-20 EOT scores, particularly in psychoactive substance dependent-abusing inpatient samples.