We have recently observed a strongly birefringent material of varying shapes and sizes in a benign breast cyst fluid specimen from a 52-yr-old woman with suspicious mammographic microcalcifications. The finding of calcium oxalate crystals in the breast cyst fluid should be regarded as a recognition of a particular type of calcification, easily overlooked with the conventional light microscopy. To date, we are unaware of any cytologic reports of this type of calcification in the breast