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 followed 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 age 68 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 (>4y) were reviewed. Distinct hospitalizations involving critical care use (ICU/CCU) were counted and associated reimbursements were assessed; repeated use was defined as 5 such hospitalizations. 54.9% of the cohort used any critical care sometime after diagnosis. Older patients were much less likely to ever use critical care (OR=0.31; comparing >90 to 68-70), even after adjustment. 31,348 patients (2.8%) were repeated users of critical care; they accounted for $3.6billion in hospital charges and $1.4billion in Medicare reimbursement. We conclude that critical care use is common in serious chronic illness and is not associated solely with pre-terminal hospitalizations. Use is uneven, and a minority of patients who repeatedly use critical care account for disproportionate costs.