A serial bone biopsy longitudinal study has been undertaken to asses the effects of Dialysis and Transplantation on uremic osteodystrophy. The clinical material consists of: normal (24 cases, 61 biopsies), patients with moderate renal failure on free diet (50 c., 61 b.), patients on Low Protein Diet (146 c., 251 b.), patients on Regular Dialysis Treatment (114 c., 256 b.), patients submitted to Transplantation (50 c., 84 b.). A significant correlation has been documented between duration of low protein diet and bone changes. In long-term dialysis patients remarkable differences have been found in "late" dialysis starting time (mean value of creatinine clearance at the beginning of treatment 2.04 +/- 1.7) in respect to "early" starting-time (mean value 13.1 +/- 3.9). In late group bone histomorphometry showed quite evident signs of secondary hyperparathyroidism and hyperosteoidism which remarkably worsened 3-4 years after dialysis. The aminoacids composition of the bone showed an increase in the total amount of aminoacids during dialysis, expecially for proline, hydroxyproline and glycine. In early group both basic values and progression rate of bone lesions were less severe. In transplanted patients with good renal function (serum creatinine less than 1.4 mg%) bone histomorphometric analysis showed a rather rapid disappearance of uremic bone lesions. A significant correlation was found between serum PTH, renal function and bone histology. On some occasions signs of secondary hyperparathyroidism were documented even after 16 months. A good correlation between inactive bone surfaces and immunosuppressive regimen was found.