This study aims to contribute to the understanding of underlying causes for the development of Internet Addiction Disorder (IAD) and assess comorbidity with other mental disorders through the analysis of data from a clinical sample of college students who presented for treatment of IAD. The clinical sample of our study has demonstrated a high percentage of comorbidity with Axis I and II disorders, while the temporal precedence of the establishment of those disorders cannot lead to specific conclusions. Half of the sample (25/50) presented with comorbidity of another Axis I disorder and 38% (19/50) with a concurrent Axis II personality disorder. The majority of Axis I disorders (51.85%) were reported before the onset of IAD, 33.3% after the onset while it was unclear in 14.81% of cases. The examination of a path model demonstrated that important contributions to the understanding of this disorder can be made through concepts from the neurobiological, trait personality paradigm, as well as from the psychodynamic defense style paradigm. Comorbid psychopathology can further exacerbate the presentation of IAD through a direct link, regardless of the underlying personality structure. The clinician treating IAD patients should complete a clinical evaluation for comorbid Axis I and II diagnoses since their presence may signify a more serious presentation.