In 25% of the samples the often applied qualitative chemical analysis of urinary stones leads to entirely wrong results with severe therapeutic consequences. The appropriate techniques for stone analysis are infrared spectroscopy and X-ray powder diffraction. These techniques make also possible the identification of iatrogenic urinary calculi. Four types of such stones were detected here, caused by modern medication. N4-acetylsulfamethoxazole, N4-acetylsulfadiazine, mefenamic acid and silicon dioxide. It is only the correct preanalytical treatment of urine samples that prevents considerable impairment of analytical results. Without precautions especially, the oxalate concentration in urine may be doubled or tripled during one day of storage.