Mental illness is common, and usually starts early in life. However, the majority of those affected never seek mental health care. The overall aim of this thesis was to increase knowledge about help-seeking young adults with mental illness in order to improve diagnostic procedures in clinical psychiatry. A group of young adult psychiatric out-patients (n=217) were consecutively invited to participate in the study between October 2002 and September 2003. Altogether 200 (92%) agreed to participate. Among them, there were 161 (80%) women and 39 (20%) men. Participants’ mean age was 22.4±1.9 years. All participants were carefully and comprehensively assessed with respect to axes I, II, IV and V in the DSM-IV. Psychiatric disorders and personality disorders were assessed using the Structured Clinical Interview for DSM-IV for axis I disorders and the Structured Clinical Interview for DSM-IV for axis II disorders. Psychosocial and environmental problems (axis IV) were evaluated through structured interviewing by a social worker and by self-assessment with a questionnaire. Professional and patient ratings on the Global Assessment of Functioning scale were compared before and after treatment. Patients also reported on the Swedish universities Scales of Personality, the Child and Adolescent Psychiatric Screening Inventory-Retrospect and the Coddington’s life event scale. Taken together, the young adult, psychiatric outpatients were characterized by an early onset of their mental disorders, by co-morbidity, by being female and by having mood or anxiety disorders. There were no significant differences between self-referred and those referred by medical professionals according to either number of current or lifetime diagnoses. Childhood onset of depression was associated with more severe symptoms, more psychosocial risk factors, and more childhood developmental delays. Axis IV psychosocial stress categories were related to the presence of axis I disorders, personality disorders, co-morbidity, and impaired functioning. Agreement between patients’ and professionals’ ratings on the GAF scale was good before treatment and excellent after treatment. In summary, the findings suggest that direct self-referral to specialized psychiatric care does not seem to be associated with overutilization of such care. Childhood onset of depression is associated with a more complex illness. The revised axis IV according to DSM-IV seems to have concurrent validity, but is still hampered by limited reliability. And finally, the results support the usefulness of the self-report GAF instrument for measuring outcome in psychiatric care.