Birefringent needlelike crystals in rosette or wheat-sheaf-like arrangements were found in pulmonary cytology specimens from 11 of 65 patients who had either sputum cultures positive for Aspergillus or histologically confirmed pulmonary aspergilloma. No crystals were found in specimens from 60 control patients with and without known fungal disease. The crystals were most often associated with A. niger infection (45.4%), followed by A. flavus (16%). Crystals were also observed in one case of A. fumigatus infection and in one case in which the species was not determined. In two cases, crystals were found more than one year before sputum cultures became positive; in one of these patients, a fungus ball was not identified by X ray until five years after the first appearance of the crystals in the sputum. Sixty-four percent of the patients with crystals also showed moderate to severe cytologic atypia. The crystals are thought to be calcium oxalate. We conclude that the presence of birefringent needlelike crystals with rosette or wheat-sheaf-like arrangements in pulmonary cytology specimens is a reliable marker for the presence of Aspergillus infection, which may be detected before cultures are positive or a fungus ball is evident on X ray.