Although categorical diagnoses are valued by clinicians and those wishing to collect homogeneous groups of patients for research projects, in the field of non-psychotic mental disorders they correspond poorly to actual clinical syndromes. Indeed, patients often satisfy several sets of categorical criteria simultaneously. A more parsimonious representation of symptomatology can be obtained by using a dimensional model for symptoms – with two correlated axes corresponding to anxiety and depression for ratings derived from the Present State Examination. The Composite International Diagnostic Interview is likely to produce a third dimension – somatic symptoms – because of a radically different symptom content. Preliminary data analysis indicates that the somatic symptoms dimension correlates with both anxiety and depression. Symptomatic individuals in particular areas of dimensional space will satisfy several criteria for categorical diagnoses, and thus display comorbidity. However, evidence that these disorders are distinct is far from persuasive, as they appear to share several common causes.