To help determine whether or not the elderly are at unusually high risk for hospital-related complications, a study was made of consecutive admissions involving 48 patients less than 65 years old (mean 50.3 years) and 174 patients more than 65 years old (mean 73.1 years). They were prospectively followed during their hospital stays, and were monitored daily. According to admission data, they were classified as : Group 1, likely to die soon after admission; Group II, likely to die within a year; Group III, likely to live longer than a year. Complications were listed under the causal factors: trauma, infection, hospital procedures, drugs, and miscellaneous. About 90 per cent of all 222 patients in both age categories were in Group III. The overall complication rates were 29 per cent for patients under 65, and 45 per cent for those over 65 (P less than 0.05). For the under-65 Group III patients, the rates were 17 per cent and 42 per cent for medical and surgical patients, respectively; for the over-65 patients, the rates were 40 per cent (medical) and 43 per cent (surgical). This indicated a major increase in complications among medical patients more than 65 years old. Procedure-related problems were most common among surgical patients under 65. For over-65 patients, the complication rates for procedure-related, trauma, miscellaneous, infection, and drug toxicity categories were fairly evenly distributed within the range of 16.8-12.8 per cent, in that order. Major psychiatric deterioration was observed in 10.7 per cent of patients over 65, but in none of the younger subjects. Drug toxicity and traumatic complications were also more common in the older patients. These data confirm the disproportionately high incidence of iatrogenic diseases in hospital patients over the age of 65. Several of these complications are amenable to control.