In 2008, the lack of a robust estimate for the proportion of patients experiencing preventable deaths in English acute hospitals was fuelling debate and hindering progress in tackling the underlying problems associated with serious patient harm. In this thesis a narrative literature review and a study of harm measures in a single acute hospital are used to guide the choice of method for a study to determine the proportion of preventable hospital deaths. A subsequent retrospective case record review (RCRR) of 1000 randomly sampled deaths from 10 English acute hospitals found the proportion of preventable deaths to be 5.2% (95% CI, 3.8% to 6.6%) which would equate to 11,859 (95% CI 8712 to 14 983) preventable deaths per year in NHS hospitals in England, 60% of whom had a life expectancy of less than 1 year. The proportion was lower than previous estimates based on US RCRR studies but consistent with a recent Dutch study which reviewed 3,983 hospital deaths. The majority of underlying problems in care were related to clinical monitoring, diagnostic error and drug and fluid problems, and 44% occurred during ward care. Problems were more likely to occur in surgical than medical patients (23.6% vs12.7%). Three-quarters were omissions, rather than commissions, in care and accumulated throughout the hospital episode. While there was a strong positive correlation between proportions of preventable deaths in hospitals and MRSA bacteraemia rates (r=0.73; p<0.02) there were no other significant associations with common measures of safety, including HSMR. Improvements are needed to reduce human error and to provide better quality of care for acutely ill older people to reduce serious harm in acute hospitals. A national mortality review process, based on this study, is to be rolled out across the NHS and will provide one mechanism for monitoring progress.