BackgroundThe public health significance of mixed anxiety–depressive disorder (MADD) and the distinctiveness of its phenomenology have yet to be established.AimsTo determine the public health significance of MADD, and to compare its phenomenology with ICD-10 anxiety, depressive, and comorbid anxiety and depressive disorders.MethodWeighted analysis of data from the Great Britain National Psychiatric Morbidity survey was conducted with a representative household sample of 8580 persons aged 16–74 years.ResultsThe 1-month prevalence of MADD was 8.8%. A fifth of all days off work in Britain occurred in this group. The symptom profile of MADD was similar to ‘pure’ ICD-10 anxiety and depression, but with a lower overall symptom count. The disorder was associated with significant impairment of health-related quality of life. Differences in health-related quality of life measures between diagnostic groups were accounted for by overall symptom severity, which remained strongly associated with health-related quality of life measures after adjusting for diagnostic group. The finding that half of the anxiety, depression and MADD cases and a third of the comorbid depression and anxiety cases grouped into a single latent class challenges the notion of these conditions as having distinct phenomenologies. Mixed presentations may be the norm in the population.ConclusionsThe data support the pathological significance of MADD in its negative impact upon population health. Dimensional approaches to classification may provide a more parsimonious description of anxiety and depressive disorders compared with categorical approaches.