Older patients may be excluded from intensive care units because of the perception that they will benefit less than younger patients. To determine if advanced age is associated with increased mortality independent of severity of illness, we compared older and middle-aged patients admitted to a medical intensive care unit. We reviewed the charts of 130 patients age 75 years or older and 135 patients age 55 to 65 admitted over a 30-month period. We controlled for severity of illness using the Acute Physiology Assessment and Chronic Health Evaluation (APACHE II) system without including points for age (APACHE IIM). The groups were similar with regard to gender, whether or not they had a private attending physician, mean APACHE IIM score, and diagnoses, except that older patients had more chronic obstructive pulmonary disease. Hospital stay was slightly longer in the older group (37 vs. 39 days, rank sum, P less than .02). Hospital mortality was significantly greater in the older group (39% vs. 51%, Chi-square P less than .05) with a crude relative risk of 1.32 (95% confidence interval [CI]: 1.01, 1.73). However, the relation of age group to mortality differed for patients with different diagnoses. When we used logistic regression to adjust for APACHE IIM, whether the patient had a private attending physician, primary admitting diagnosis, or presence of cancer, older patients did not have a significantly greater risk of dying (adjusted relative risk, 1.05; 95% CI: 0.97, 1.12). When pulmonary artery catheterization was added to the model, it independently predicted mortality adjusted relative risk, 1.47; 95% CI: 1.05, 2.06.(ABSTRACT TRUNCATED AT 250 WORDS)