Crystalluria is the excretion of microscopic crystals by urine.
The solute do not form crystals in normal urine, so the microscopic detection of crystals in urine is an abnormality. The reason may be innocuous such as in insufficent water intake, in case of some medicaments, of simply evaporation of water before microscopy. In rare cases urinary crystals indicate severe disorders. In some of these cases the diagnosis can be even mad by the picture of those crystals.