To assess the importance of multiple chronic diseases (co-morbidities) to how people feel about their lives generally. To do this, we studied the associations between co-morbidities and indicators of quality of life and/or psychological distress. Analysis of unit record cross-sectional data from Australian national surveys for the population aged 20 years or more. Identification of an appropriate indicator of multiple chronic diseases (i.e. co-morbidities). Use of logistic regression techniques to study associations between: (1) co-morbidities and demographic, socio-economic and risk factor variables; and (2) quality of life (general and psychological distress) and demographic, socio-economic and health status indicators. Older people, obese persons, women, persons with low socio-economic status and those living alone had significantly greater probability of having three or more chronic illnesses than did other people aged 20+ years (p<0.0001). Also, people with co-morbidities and/or with poor self-rated general health, those living alone, people with low educational qualifications and persons with low socio-economic status were more likely to feel dissatisfied, unhappy or terrible about their lives and to have moderate, high or very high psychological distress scores than the rest of the 20+-year-old population (p<0.0001). Multiple chronic diseases were found to have a considerable negative impact on quality of life, not only in terms of how people felt about their lives generally, but also in terms of the extent of their psychological distress.