Relative low serum levels of parathormone (PTH) and low incidence of secondary hyperparathyroidism have been reported in diabetic uremic patients. The pathogenesis of this reported resistance to uremic secondary hyperparathyroidism in diabetes remains controversial. We have measured the serum C-terminal parathormone (C-PTH) renal phosphorus threshold (TmPO4) and nephrogenous cyclic AMP (N-cCAMP), in 2-hour urine collection in 22 patients with diabetic nephropathy with moderate chronic renal failure and in 27 controls with similar creatinine clearance values (18.16 +/- 9.14 and and 19.1 +/- 8.47 ml/min). In spite of the lower levels of serum C-PTH (1.07 +/- 0.43 ng/ml) diabetic patients exhibited an increased phosphaturia (TmPO4: 1.97 +/- 0.9 mg/100 ml GFR) when compared with the control group (C-PTH: 2.01 +/- 1.17 mg/ml, and TmPO4: 2.5 +/- 0.7 ml GFR). When the C-PTH values were plotted against the logarithm of creatinine clearance values, both groups showed a significant linear relationship reflecting the progressive increase in PTH when GFR fell. This progressive parathyroid stimulus was also present in diabetic patients but in a lower intensity. We believe that increased phosphaturia in diabetics with moderate chronic renal failure may be a major factor in precluding the appearance of secondary hyperparathyroidism in these patients once they reach the dialysis and transplantation programs.