AbstractThree cytologic urine specimens from separate patients seen over a period of 3 mo were prepared by the Papanicolaou method. They contained crystals (uric acid type in two and magnesium ammonium phosphate in one) that incorporated variable amounts of organic (mucoprotein) matrix; many appeared by light microscopy to be made exclusively of matrix. Scanning electron microscopy performed in one of the specimens containing uric acid crystals suggested that the matrix forms were in progressive stages of mineralization. These cases, plus a similar one reported recently by us, demonstrate that the detection by urine cytology of organic matrix incorporated in the structure of urinary crystals is not rare and that the Papanicofaou staining method facilitates such detection.