The increase in the immunocompromised population and the incidence of invasive aspergillosis (IA) are leading to an overinterpretation of the potential clinical significance of many isolates of Aspergillus fumigatus. Our work prospectively assesses the workload of the isolation of A. fumigatus and its clinical significance in the microbiology laboratory of a large teaching hospital. During a 3-year period, all patients from whom A. fumigatus was isolated were prospectively monitored and classified as having IA or "nonsignificant" disease. A point score based on the prediction of five easily obtained laboratory and clinical parameters was applied. We found 404 A. fumigatus isolates in 260 patients (1/1,000 microbiology laboratory samples; 2.1 patients/10,000 admissions). A total of 90 isolates (22.3%) were from patients with IA. Of the 260 patients, 31 (12%) had invasive disease (IA), and the remaining 229 had "nonsignificant" disease. A score based on points for five parameters was applied to our population. It was constructed as follows: "sample obtained by invasive procedures" (1 point), "presence of two or more positive samples from the same patient" (1 point), "leukemia" (2 points), "neutropenia" (5 points), and "corticosteroid treatment" (2 points). Patients with a score of 0 had only a 2.5% probability of IA. Those with a score of 1 or 2 had an increased probability of 10.3%. The probabilities rose to 40% and 70%, respectively, for patients with a score of 3 or 4 or a score of > or = 5. A simple score based on five easily available parameters may be of help to microbiologists and clinicians to predict the risk of IA.