We studied the serum albumin level within 48 hours of hospitalization for acute illness to predict in-hospital death, length of stay, and readmission in 15,511 patients older than 40 years. Patients with low serum albumin levels (less than 34 g/L), who made up 21% of the population, were more likely to die, had longer hospital stays, and were readmitted sooner and more frequently than patients with normal albumin levels. The in-hospital mortality was 14% among patients with low albumin levels, as compared with 4% among patients with normal levels. Although the serum albumin level was a nonspecific marker, it was a stronger predictor of death, length of stay, and readmission than age. We conclude that the serum albumin level on admission is an important variable that should be incorporated in severity-of-illness measures based on physiologic indexes.