An improved prognostic staging system is needed for patients with the acquired immunodeficiency syndrome (AIDS). To construct such a system, we analyzed the course of 117 consecutive adults who received a diagnosis of AIDS at Yale-New Haven Hospital from 1981 through 1987. The staging system was developed from the data on the first 76 patients, confirmed in the remaining 41 patients, and then applied to the entire cohort. The staging system, which is based on physiologic deficits rather than demographic or diagnostic features, gives one point for each of the following: severe diarrhea or serum albumin level under 2.0 g per deciliter, any neurologic deficit, arterial oxygen tension of 50 mm Hg or less, hematocrit below 30 percent, lymphocyte count below 150 per microliter, white-cell count below 2500, and platelet count below 140,000. The total score determines the presence of Stages I (0 points), II (1 point), or III (2 to 7 points). The three stages had distinctive prognostic gradients in our cohort. For patients in Stages I, II, and III, the median survival times were 11.6, 5.1, and 2.1 months, respectively, with one-year survival rates of 50, 30, and 8 percent. When the staging system was tested with a proportional-hazards model, no other descriptive or laboratory variable added any additional predictive power. Although this new staging system requires further validation in other populations, we believe it will be useful in evaluating new therapies and improving the precision of prognosis in patients with AIDS.