Hyperphosphatemia leads to the development of osteitis fibrosa in patients with chronic renal failure. In contrast, crippling osteomalacia may appear in uremic patients who are hypophosphatemic or aluminum intoxicated or who undergo total or subtotal parathyroidectomy. Thus, strict phosphorus control by use of aluminum-containing gels may ameliorate renal osteitis fibrosa, but may potentiate the development of osteomalacia. To evaluate this possibility, we compared the bone histologies of 10 chronic renal hemodialysis patients who consistently maintained predialysis phosphorus levels between 4-5 mg/dl (Strict-P) to those of 46 randomly selected dialysis patients (Random-P). We found that the Strict-P group had lower circulating immunoreactive PTH (P less than 0.02) and alkaline phosphatase (P less than 0.05) levels and, as expected, less evidence of hyperparathyroid bone disease. On the other hand, the Strict-P patients had osteomalacia, as evidenced by moderate osteoid accumulation and reduced capacity of bone to assume a fluorescent tetracycline label. Furthermore, all Strict-P patients had histological evidence of bone aluminum accumulation. We conclude that maintenance of normal serum P levels with aluminum-containing gels in hemodialysis patients prevents severe hyperparathyroid bone disease. Such treatment, however, is also attended by a moderate degree of aluminum-associated osteomalacia.