Several reports are available in the literature related to the chemistry and structure of renal calculi [1, 2]. Analytical techniques such as photomicroscopy, polarization microscopy, infrared spectroscopy, chemical tests, chromatography, X-ray diffraction analysis and scanning electron microscopy have been mostly used for such studies [2, 3]. There is great interest in the treatment of kidney stones by in vivo fragmentation, by using both invasive and non4nvasive (such as sonic and ultrasonic) methods, and laser fragmentation methods [4, 5]. Surgically treated stones comprise approximately 30% of all the stones, which are mostly composed of calcium oxalate. The calculi of other compositions are generally treated by chemical methods [6]. The classical chemical analysis of calculi necessarily involves the initial disintegration of the stone into powder form or dissolution of the stone powder in a suitable solvent for subsequent qualitative and quantitative analysis. Such an approach rules out any attempt to investigate the in situ and de facto crystalline topography of the stone in its native state. The recent introduction of acoustic methods of kidney stone fragmentation has highlighted the current lack of data regarding the mechanical properties of such calculi [7]. For all of these methods, a knowledge of the mechanical properties, such as the hardness, is used in developing optimized frequencies and intensities for stone disintegration. The work reported here was aimed at the investigation of such basic mechanical data, beginning first with a study of the (dry) hardness of such calculi, recognizing that environmental effects may also play a role in the hardness of such calculi [8]. The stone compositions are complex and very diverse [9, 10], and were therefore studied in detail. This letter also reports the range of microhardness of dry renal calculi as it varies with gross chemical composition. The investigation was made on ten samples. All specimens were mounted in cold-curing expoxy resin and were ground by using 600-grit SiC paper and polished with 0.05/~m alumina powder. After dessication for 24 h, over anhydrous CaSo4, the microhardness of the calculi was then measured by using a Knoop indentor (Zwick 3212). Indentations were made in both the parallel and perpendicular directions relative to any observable stone laminations, in order to determine possible anisotropy effects. Ten tests were made on each stone in each orientation. All indentations were made by using 50g loading each time for 30sec. The composition was determined by a Jeol JSM-35CF scanning electron microscope. Table I shows the average value of the hardness of the ten samples and the relative element composition of the calculi. The standard deviations are given in parentheses for each hardness range. The hardness values of all types of calculi are lower than the values for the pure relative (minerals) elemental composition. Thus, with the decrease in calcium content in the calculi, there is a decrease in hardness. The hardness for the dry calculi was found to vary from 16 to 125 kg mm -2 and the calcium contents were found to vary from 100 to 50.49wt %. No quantitative determination of the percentage matrix is available for these stones at this stage. The hardness was measured for different types of calculi having calcium percentages of 50.49, 56.28, 60.10, 66.63, 67.59, 79.56, 95.57, 97.04 and 100. The results of perpendicular and parallel measurements indicate that a small increase in hardness is found with increasing percentage of calcium. In this study the stone with a hardness of 125kgmm -2 was found to be the hardest. The hardness ranges overlap in several cases, as was expected because different calculi were composed of the same elements, albeit to different degrees. Here the ranges indicate the range of mean values obtained from each set of measurements at different composition percentages variation between parallel and perpendicular orientations are within 5 to 10%. Thus, the calcium