OBJECTIVES: The number of deaths occurring in private homes in England and Wales had been rising for years, increasingly rapidly from 2020. Media stories and research linked decomposing bodies found in private homes with pandemic-related social isolation. We aim to explore whether these incidents are one-offs or part of a wider trend. DESIGN: Descriptive analysis of publicly available Office for National Statistics (ONS) data. SETTING: England and Wales. PARTICIPANTS: All residents of England and Wales, 1979 to 2020. MAIN OUTCOME MEASURES: Using data from the Office for National Statistics, we calculate European Age Standardised Rates for deaths coded as R98 ('unattended death') and R99 ('other ill-defined and unknown causes of mortality') in the 10th version of the International Classification of Diseases (ICD-10), and the corresponding codes in ICD-9, by sex and age group from 1979 (when ICD-9 began) to 2020. These are proxy markers for deaths where decomposition precludes attribution of a specific cause at postmortem. RESULTS: While mortality from all other causes decreased from 1979 to 2020, the opposite was seen for deaths from R98 and R99 (or 'undefined deaths'), with men more affected than women. There was a sharp rise in these deaths in both sexes but in men particularly in the 1990s and 2000s, coinciding with a time when overall mortality was rapidly improving. CONCLUSIONS: The increase in people found dead from unknown causes suggests wider societal breakdowns of both formal and informal social support networks. They are concerning and warrant urgent further investigation. We call on national and international authorities to consider measures that would make it possible to identify these deaths more easily in routine data.