Cultures and histochemical stains for fungi were performed on concentrated, cytocentrifuged bronchoalveolar lavage (BAL) samples from 82 immunocompromised patients undergoing bronchoscopic evaluation of new pulmonary infiltrates. Aspergillus hyphae were identified in 9 of 17 BAL samples from patients with invasive pulmonary aspergillosis and from 3 of the remaining 65 study patients without this diagnosis. Thus, the presence of Aspergillus hyphae in BAL samples had a 53% sensitivity, 97% specificity, and 75% positive predictive value for the diagnosis of invasive pulmonary aspergillosis. BAL fungal cultures were positive in only 4 of 17 cases (23% sensitivity). A combination of fungal stains and cultures yielded a diagnostic sensitivity of 58% and a specificity of 92%. Results of routine fiberoptic bronchoscopy procedures in the 17 patients with aspergillosis who had BAL and in a retrospectively identified group of 10 patients with invasive aspergillosis who had only routine bronchoscopy were tabulated. Routine bronchoscopy procedures yielded a diagnosis in 5 of 27 cases (22% overall yield), with washings diagnostic in 4 of 27 cases (15%), and transbronchial biopsies positive histologically in 2 of 11 cases (18%). BAL is a valuable first procedure for diagnosing invasive pulmonary aspergillosis in the compromised host.