Objectives: There is growing evidence that the relationship between body mass index (BMI — defined as weight in kilograms divided by height in metres squared) and patient outcomes is age-dependent; specifically, a raised BMI may have a protective effect in older adults. This has been demonstrated clearly in the community setting; less clear is the effect of age on this relationship in the inpatient setting. Design, setting, participants and measurements: Retrospective analysis of 22,903 electronic records for patients discharged from two large public hospitals in South Australia between January 2015 and September 2018 inclusively. Records were retained if the patient’s height and weight had been recorded during the admission, BMI was between 10–99 kg/m2 and it was the patient’s first admission during that time. Patients were grouped by BMI (<18.5 kg/m2 (“underweight”), 18.5–24.9 kg/m2 (“normal weight”), 25–29.9 kg/m2 (“overweight”), >30 kg/m2 (“obese”)) and age (18–59 years, 60–79 years, > 80 years); for each group we measured the relative stay index (RSI) (actual length of stay divided by predicted length of stay), death in hospital and composite adverse outcome after discharge (unplanned readmission within 30 days and/or death within 30 days). Results: Underweight patients across all age groups generally experienced significantly poorer outcomes compared to those not underweight. In those aged 18–59 years there were no significant differences in outcomes between the normal weight, overweight and obese groups. In those aged 60–79 years overweight patients had a significantly reduced risk of RSI > 2 compared to those of normal weight (p=0.014), and both overweight and obese patients had a significantly reduced risk of adverse outcome after discharge when compared to those of normal weight (p=0.028 & p=0.009 respectively). In those aged 80 years or older, both overweight and obese patients had a significantly reduced risk of adverse outcome after discharge when compared to those of normal weight (p=0.028 & p=0.013 respectively), and obese patients had a significantly reduced risk of inpatient mortality and RSI >2 when compared to those of normal weight (p=0.027 & p=0.037 respectively). Conclusion: A BMI > 25 kg/m2 in older patients is associated with reduced risk of prolonged admission, inpatient mortality and adverse outcomes following discharge. This adds to growing evidence that age-specific BMI guidelines are required for adults because the healthiest BMI in the older hospital patient is seemingly not in the range 18.5–24.9 kg/m2.