MATERIALS AND METHODS

Kidneys from 100 randomly selected autopsies were bisected, with the incision beginning at the hilus; the collecting systems were opened and all papillae were examined. Two sections of each kidney were taken for microscopic examination, including a calix, a pyramid with papilla and the full-thickness of the cortex. After 24 hours of fixation in buffered neutral formalin, the sections were processed in a routine manner and stained with hematoxylin and eosin. The alizarin red stain for calcium was done in each case. The medulla was examined with the low power objective for basophilic deposits representing calcium, and each section was graded as follows: grade 1-1 to 3 focal small deposits of basophilic material, grade 2--more than 3 deposits which were usually larger and often involved basement membranes and grade 3-multiple deposits often with extensive involvement of tubular basement membranes. The highest grade observed in any one section was taken as the over-all grade for that patient. Material for electron microscopy was selected from cases with a postmortem interval not exceeding 4 hours. Tissue from papillary tips was fixed for 4 hours in cacodylate-buffered 3 per cent glutaraldehyde. The blocks were post-fixed in 2 per cent osmium tetroxide and embedded in Dow epoxy resin 334. Sections were cut with an