Aluminum in undemineralized, methyl methacrylate-embedded iliac bone from 16 hemodialyzed renal patients and six normal controls was stained by a modification of the aluminon method and quantitated histomorphometrically, and the results were compared with the aluminum values measured by atomic absorption spectrophotometry. There was a high degree of correlation between the two techniques (r = 0.955, p less than 0.0001). The amount of stained aluminum also showed a statistically significant relationship with the amount of bone that is not mineralized (r = 0.840, p less than 0.001), with the TM (r = 0.841, p less than 0.001), and with osteoid width (r = 0.771, p less than 0.0001). The aluminum was found as bright red bands mainly in the junction of mineralized bone and osteoid but was also observed in cement lines of mature bone, on neutral surfaces, and surrounding the osteocytes. Analysis of serial sections for aluminum and tetracycline showed that a majority of the sites with aluminum did not take up tetracycline, implying defective mineralization. Occasionally tetracycline uptake was observed in such places, suggesting that the aluminum "block" might be overcome eventually. The presence of aluminum in cement lines of mature bone supports this hypothesis. The histochemical staining of aluminum provides a quick and easy method for identifying, localizing, and quantifying aluminum within the bone and may prove to be a useful tool in the study of the mechanisms of metabolic bone disease.