Four factors influence ICU outcome: age, previous health status (HS), severity of disease, and diagnosis. To assess the specific influence of each factor, 3,687 patients from 38 French ICUs were studied. For each patient, the following were noted: age, Simplified Acute Physiologic Score (SAPS), previous HS, diagnosis, ICU group (medical [M], surgical unscheduled [S], surgical scheduled or elective [E]), and the immediate outcome. The multivariate analysis ranks the factors in the following order: SAPS, age, ICU group, and previous HS. Diagnosis played a key role in prognosis, e.g., with SAPS score between 10 and 15 points, mortality was 0 for drug overdose, 12% for chronic obstructive pulmonary disease, and 38% for cardiac shock. For most patients, it was difficult to assign only one diagnosis; 37% were given one diagnosis, 39% two diagnoses, 17% three diagnoses, and 7% four and more diagnoses. Using ICU group instead of diagnosis, the mean death rate for groups M, S, and E was 20%, 27%, and 5%, respectively (p < .001). Our study shows that each of the four factors influences the immediate outcome. It is thus possible to describe and classify ICU patients by this method, but the individual prognosis remains imprecise. (Crit Care Med 1989; 17:305)