To evaluate the value of a positive nasal swab for Aspergillus in the diagnosis of invasive aspergillosis, we prospectively evaluated nasal colonization in 173 episodes of neutropenia in 92 patients with hematological malignancies. Weekly nasal swabs were taken, and the patients were followed until death or resolution of neutropenia. The outcome variables were the development of invasive aspergillosis, empirical antifungal therapy and death. In 31 episodes of neutropenia (18%) there was at least one positive nasal swab for Aspergillus sp. Only two patients developed invasive aspergillosis, both with a positive nasal swab (p = 0.03). The positive and negative predictive values of a nasal swab were 6.4% and 100%, respectively. There was no difference between patients with positive or negative swabs regarding antifungal therapy or death. In this population of patients, a nasal swab for Aspergillus sp. had a low positive predictive value and a high negative predictive value for invasive aspergillosis.