Background: There is concern that people seeking treatment over the Internet for anxiety or depressive disorders may not resemble the general population or have less severe disorders than patients attending outpatient clinics or cases identified in community surveys. Thus the response to treatment in Internet based trials might not generalize. & Methodology: We reviewed the characteristics of applicants to an Australian Internet-based treatment clinic for anxiety and depression, and compared this sample with people from a national epidemiological survey and a sample of patients at a specialist outpatient anxiety and depression clinic. Participants included 774 volunteers to an Internet clinic, 454 patients at a specialist anxiety disorders outpatient clinic, and 627 cases identified in a national epidemiological survey. Main measures included demographic characteristics, and severity of symptoms as measured by the Kessler 10-Item scale (K-10), the 12-item World Health Organisation Disability Assessment Schedule second edition (WHODAS-II), the Penn State Worry Questionnaire (PSWQ), the Body Sensations Questionnaire (BSQ), the Automatic Cognitions Questionnaire (ACQ), the Social Interaction Anxiety Scale (SIAS) and the Social Phobia Scale (SPS). & Principal Findings: The severity of symptoms of participants attending the two clinics was similar, and both clinic samples were more severe than cases in the epidemiological survey. The Internet clinic and national samples were older and comprised more females than those attending the outpatient clinic. The Internet clinic sample were more likely to be married than the other samples. The Internet clinic and outpatient clinic samples had higher levels of educational qualifications than the national sample, but employment status was similar across groups. & Conclusions: The Internet clinic sample have disorders as severe as those attending an outpatient clinic, but with demographic characteristics more consistent with the national sample. These data indicate that the benefits of Internet treatment could apply to the wider population.