Dynamic skeletal histomorphometry was performed in 94 unselected patients receiving maintenance dialysis for chronic renal failure. An attempt was made to correlate the results with the clinical, biochemical and radiological findings. Skeletal histology was abnormal in each case. Hyperparathyroidism was present as the only abnormality in 18 patients and osteomalacia in 26; 50 patients showed both abnormalities. Osteomalacia, in contrast to hyperparathyroidism, increased in prevalence and severity with the duration of dialysis and with bone aluminum content. The majority of patients had histological osteosclerosis. It was impossible to predict either the nature or the severity of the histological lesions on the basis of symptoms and physical signs or on the basis of most biochemical parameters (including serum concentrations of three vitamin D metabolites). Serum alkaline phosphatase values and serum immunoreactive parathyroid hormone (iPTH) concentrations were positively correlated with the severity of histological hyperparathyroidism. Subperiosteal erosions of the phalanges were associated with severe histological hyperparathyroidism in each case but this radiological sign was absent in 66% of patients with histological hyperparathyroidism. Radiological osteosclerosis was associated with severe histological osteomalacia in each case, but this radiological sign was absent in 87% of patients with histological osteomalacia. No other radiological sign proved a reliable guide to the underlying skeletal histology. In the majority of dialysis patients, a skeletal biopsy is required for an accurate diagnosis of the nature and severity of azotemic osteodystrophy.