Background: Insight affects adherence and treatment outcome and relates to cognitive impairment and psychopathology. We investigated the relationship of insight with cognition in patients with major depression, schizophrenia and bipolar disorder in acute psychiatric care, long-term inpatient, and outpatient settings. Methods: Eighty-one patients (women, 59.5%; age, 45.9 ± 13.5 years; 27 in each setting group; 33.3% with DSM-IV bipolar disorder, 39.5% with unipolar major depression, and 27.2% with schizophrenia) underwent the Wisconsin Card Sorting Test (WCST) to test flexibility, clinician-rated Scale to Assess Unawareness of Mental Disorder (SUMD), and self-rated Insight Scale (IS) to assess insight/awareness. Results: Poor performance on the WCST correlated with higher SUMD scores such as current psychiatric illness unawareness, impaired symptom attribution, unawareness of medication effect, or of social consequences, but not with IS scores. The latter correlated with days on continuous treatment. Patients receiving psycho-education showed greater symptom awareness compared to patients treated with drugs alone. Cognitive flexibility and diagnostic category did not correlate. Poor insight corresponded with severe mental illness, particularly acute psychosis. Conclusions: Treatment setting specificity reflects psychopathology and severity. Insight is inversely proportional to illness severity and cognitive flexibility, which is also affected by psychopathology. Limitations comprise group heterogeneity, cross-sectional design, and limited sample size.