Review of the bronchoalveolar lavage specimens from 326 patients resulted in the identification of Alternaria in 28 (8.6%) of the specimens. On Papanicolaou-stained Millipore filters, the most common finding was a yellow-brown-pigmented muriform conidium with characteristic transverse and longitudinal septations. Four of the patients had floccose branched and septated hyphae of Alternaria in addition to conidia. Budding yeast or yeast forms were also present in the lavage fluid of 14 of the patients with Alternaria. Two patients had concurrent Pneumocystis carinii pneumonia, and one patient had cytomegalovirus pneumonitis. No patient developed clinical features of systemic Alternaria infection, and autopsy of four patients did not reveal pneumonia. Alternaria conidia in a bronchoalveolar lavage fluid will usually represent laboratory contaminants or nonpathogenic saprophytes, and their significance lies in distinguishing them from other fungi. However, the expanded use of immunosuppressive therapy and the increasing prevalence of acquired immune deficiency syndrome may render such saprophytes clinically important.