Presence of crystals in urine sediment is not an infrequent finding, and its observation does not have any especial clinical signification in most situations. Some factors as pH, temperature, and emission time of the urine may play a part in their formation. Nevertheless, in some diseases such as congenital anomalies or severe liver pathology, some types of crystals (cystine, tyrosine, and leucine) are quite rare and its identification is very important. In urinary cytopathology, crystals such as uric acid, calcium oxalate dihydrate, and ammonium magnesium phosphate (struvite) are among those that may be frequently observed in urine sediment. Taking into account its morphology, they are usually named as ‘‘lemons’’ (uric acid), ‘‘envelopes’’ (calcium oxalate dihydrate), and ‘‘coffin linds’’ (struvite). Here, a type of crystal (calcium oxalate monohydrate), not as frequent as those mentioned earlier is showed (Fig. C-1). The image belongs to a voided urine smear from a patient suffering acute renal failure. Calcium oxalate monohydrate crystals vary in size and, depending on their position, they give a different projection showing the typical ‘‘dumbbell’’ shape or round–oval morphology with a central depression (they are also described as ‘‘hour-glass" or ‘‘sheaf’’). Factors such as excessive consumption of certain aliments rich in oxalates (spinaches, asparagus, tomatoes, etc), a decrease in the urinary pH, and a scant diuresis may contribute to its development. The presence of calcium oxalate crystals (mono or dihydrate) in the urine sediment does not mean necessary the existence of calculi in the urinary tract. In fresh and unstained urine samples, and depending of their size, this type of crystals must be differentiated from erythrocytes, yeast, and parasitic ova, especially Enterobius vermicularis. However, its high refraction index can solve the problem.