Background Several studies have shown that general practitioners (GPs) under-diagnose common mental disorders, and that training courses hardly improve this practice. The influence of GPs' self-perceived ability to recognize the severity of such disorders on these facts has not been investigated. This study explores: 1) GPs' perceived ability to recognize major depressive episode (MDE) and generalized anxiety disorder (GAD) in their patients; 2) The GPs' observed ability to recognize severity of these disorders; and 3) If the observed ability to recognize severity is associated with their perceived ability. Methods In a cross-sectional design 40 Norwegian GPs examined 15 – 28 patients each (total N = 724). The GPs' rated their perceived ability to recognize MDE and GAD on a four-point Likert-scale. The GPs' observed ability to recognize severity was defined as the mean of the correlations between the GPs rating of Clinical Global Impression-Severity Scale and the diagnostic reference standards for MDE and GAD filled in by patients. Results Twenty-two GPs considered their perceived ability to recognize MDE as rather good, and the other 18 as moderate/bad. For GAD 12 GPs' perceived their ability as rather good, while 28 considered their ability to be moderate/bad. The observed ability to recognize severity concerning MDE was 0.63 and concerning GAD 0.45. There was no significant association between GPs' perceived and observed abilities to recognize MDE (p = 0.19) and GAD (p = 0.34) Conclusion This study found a discrepancy between the GPs' perceived and observed ability to recognize common mental disorders. The lack of association between GPs' perceived and observed ability to recognize such disorders indicate low understanding of own recognition abilities. This might contribute to explain the low effectiveness of interventions aimed to increase GPs' abilities to recognize mental disorders.