Microscopic crystals were found by SEM analysis in kidneys of stone forming patients as well as in kidneys of non-stone formers. In most samples calcifications were intratubular in the collecting ducts near the papillary tip. 1-2 micron sized particles lay in a gelatinous substance on the epithelium. The only difference in crystal composition is the higher phosphate content in the kidneys of the non-stone forming group. A correlation between the element concentration of the tissue and the occurrence of microliths is given by the cadmium content of the kidney samples. Both groups showed higher cadmium values in the cortex or medulla when crystals were found in the corresponding papillary samples. Urine supersaturation with respect to calcium oxalate was higher in the stone forming group caused by a higher calcium and oxalate but a lower magnesium excretion.