BackgroundComorbidity is common. National Institute for Health and Clinical Excellence (NICE) guidelines have been created to make best use of resources to improve patients' quality of life but do not currently take account of comorbidity. The effect of multiple chronic conditions with regard to health-related quality of life (HRQoL) is poorly researched. Criticisms of previous research have been due to patient-defined chronic conditions, lack of quantification of the effects of confounding factors, selection of affected patients only, small sample sizes, and upper age limits. AimThis study aims to address these issues, looking into the impact of combinations of chronic conditions on HRQoL. Design of the studyParticipants filled in a questionnaire containing general health information, specific respiratory questions, and the EQ-5D measure of HRQoL. The questionnaires were then matched up to their GP records to obtain their disease status for six common chronic diseases (asthma, chronic obstructive pulmonary disease, ischaemic heart disease, hypertension, diabetes, and cerebrovascular disease). MethodData from a mailed questionnaire were analysed from 5169 patients aged >16 years from two general practices in Wythenshawe, Manchester in 2004. Completion of the questionnaire was taken to indicate consent to participate. ResultsSignificant correlations were found between a lower HRQoL and increasing numbers of chronic conditions (P<0.001), increasing age, possible obstructive airway disease, lack of higher education, smoking, and female sex. These all remained significant following regression, except for sex, with number of chronic conditions being a strong predictor of the weighted health state index score, EQ-5Dindex (coefficient = -0.079, P<0.001). ConclusionIncreasing numbers of chronic conditions have a strong negative effect on HRQoL.Keywords chronic disease; comorbidity; EQ-5D; health related quality of life. INTRODUCTIONImprovements in living conditions, together with advances in medical science, have led to an increased number of patients living with chronic conditions. Studies indicate that comorbidity is the normal state of affairs, especially in patients aged >65 years, 1 and 50% of patients with a chronic disease have more than one.2 Within the UK healthcare system, there has therefore been a shift in the way in which patients are treated, evidenced by the Department of Health recognising the importance of long-term conditions, 3 and by The Royal College of General Practitioners including the ability to provide person-centered care as a core competence for GPs. 3,4 This, in addition to numerous choices in treatments and the limitations of a publicly funded health service, has made quality of life measurement a hugely relevant issue in 2009.Priorities in health care must be set, and it is now generally accepted that one of the most important considerations should be the improvement in patients' health-related quality of life (HRQoL) with a treatment, 5,6 rather than relying solely on change meas...