Despite its expense and importance, it is unknown how common critical care use is. We describe longitudinal patterns of critical care use among a nationally representative cohort of elderly patients monitored from the onset of common serious illnesses. A retrospective population-based cohort study of elderly patients in fee-for-service Medicare is used, with 1,108,060 Medicare beneficiaries at least 68 years of age and newly diagnosed with serious illnesses: 1 of 9 malignancies, stroke, congestive heart failure, hip fracture, or myocardial infarction. Medicare inpatient hospital claims from diagnosis until death (65.1%) or fixed-right censoring (more than 4 years) were reviewed. Distinct hospitalizations involving critical care use (intensive care unit or critical care unit) were counted and associated reimbursements were assessed; repeated use was defined as five or more such hospitalizations. Of the cohort, 54.9% used critical care at some time after diagnosis. Older patients were much less likely to ever use critical care (odds ratio, 0.31; comparing patients more than 90 years old with those 68-70 years old), even after adjustment. A total of 31,348 patients (2.8%) were repeated users of critical care; they accounted for 3.6 billion dollars in hospital charges and 1.4 billion dollars in Medicare reimbursement. We conclude that critical care use is common in serious chronic illness and is not associated solely with preterminal hospitalizations. Use is uneven, and a minority of patients who repeatedly use critical care account for disproportionate costs.