This article offers national estimates of the proportions of hospital inpatient cases and cost for adult, nonmaternal patients who have multiple chronic conditions. The authors employ a refined classification of chronic versus acute conditions, collapsed to no more than one condition per distinct category of condition. The number of different chronic conditions provides a simple measure of complexity, differing from measures of severity of illness that pertain to a particular episode of treatment. A multivariate regression finds that the number of chronic conditions is an independent influence on hospital cost per case, controlling for other key determinants. Patients with complex illness (e.g., 3+ or 5+ chronic conditions) have a disproportionately large effect on hospital cost per year. The identification of patients in the hospital with complex illness can help in targeting new covered services in a health plan or in risk adjusting health plan premiums. Current policies and demonstrations for the Medicare program may not be sufficient to address complex illness.