A 56-year-old man presented with massive, left-sided pleural empyema. Sputa, pleural fluids, a bronchial washing and a bronchial biopsy revealed acute inflammatory exudate and numerous birefringent calcium oxalate crystals. One pleural fluid also showed occasional mycelia and rare conidiophores of Aspergillus niger. The fungus was abundantly cultured from all cytology specimens. Other oxalosis-related conditions were not identified in this patient. The finding of calcium oxalate crystals associated with a background of acute inflammatory cells in cellular samples of respiratory secretions and pleural fluid should be regarded as a clue to the diagnosis of infection with Aspergillus niger.