ABSTRACT Objective The incidence and cost of complications occurring in older and younger inpatients were compared. Design Secondary analysis of hospital-recorded diagnosis and costs for multiday-stay inpatients in 68 public hospitals in two Australian states. Main outcome measures A complication is defined as a hospital-acquired diagnosis that required additional treatment. The Australian Classification of Hospital- Acquired Diagnoses system is used to identify these complications. Results Inpatients aged >70 years have a 10.9% complication rate, which is not substantially different from the 10.89% complication rate found in patients aged <70 years. Examination of the probability by single years, however, showed that the peak incidence associated with the neonatal period and childbirth is balanced by rates of up to 20% in patients >80 years. Examining the adult patient population (40e70 years), we found that while some common complications are not age specific (electrolyte disorders and cardiac arrhythmias), others (urinary tract and lower respiratory tract infections) are more common in the older adult inpatient. Conclusion For inpatients aged >70 years, the risks of complications increase. The incidence of hospitalacquired diagnoses in older adults differs significantly from incidence rates found in younger cohorts. Urinary tract infection and alteration to mental state are more common in older adult inpatients. Surprisingly, these complexities do not result in additional costs when compared with costs for the same complications in younger adults. Greater awareness of these differing patterns will allow patient safety efforts for older patients to focus on complications with the highest incidence and cost