BACKGROUND: Current government policy is directed towards reducing inequalities in health. General practice is increasingly the focus for health care provision in the United Kingdom and it is therefore important to understand the relationship between socioeconomic factors and health at the general practice level. A deprivation measure was used for the area in which the practice is located and two key priority areas were focused upon: mental health and heart disease. AIM: To explore the relationship between area deprivation and the prevalence of treated heart disease and mental illness in England and Wales. DESIGN OF STUDY: Analysis of data from medical records from practices contributing to the General Practice Research Database. SETTING: A total of 211 practices located in England and Wales. METHOD: The data from the practices were used to examine deprivation differentials in treated disease prevalence rates. RESULTS: There are strong deprivation gradients (highest prevalence in the most deprived category) for treated depression, anxiety, schizophrenia, coronary heart disease and non-insulin-treated diabetes. In general the deprivation differentials are wider in the middle of the age range. Women aged 35 to 64 years in the most deprived areas had rates of treated coronary heart disease two to three times those in the least deprived areas. Men aged 25 to 44 years in the most deprived areas had rates of treated depression 50% higher than those in the least deprived areas. CONCLUSIONS: This study has identified wide deprivation differentials in the prevalence of treated mental illness and heart disease. If all of England and Wales experienced the rates of the least deprived fifth of the population then the number of people being treated for depression, anxiety, and coronary heart disease would fall by 10%, 16%, and 11% respectively.