Adynamic bone is being found with increasing frequency in dialysis patients. Little is known about its aetiology, and even less about its natural history. We found 10 cases of asymptomatic adynamic bone among a group of 32 continuous ambulatory peritoneal dialysis patients, most of whom had never been exposed to aluminium-containing phosphate binders. Compared to the remaining 22 patients, they had an older mean age (54 ± 11.4 vs. 42 ± 11.8 years; p < 0.05), probably a longer pre-dialysis duration of renal failure (10.9 vs. 7.1 years), higher mean ionized calcium (1.30 ± 0.04 vs. 1.15 ± 0.02 mmol/l; p < 0.01), and lower mean intact parathyroid hormone (31.5 vs. 200.3 pg/ml; p < 0.001). The bone density was not different between the two groups, but 9 of the 10 adynamic patients had significant vascular calcification seen on plain radiology as compared with only 7 of 20 in the comparison group (p < 0.05). Follow-up of the adynamic patients showed a close association with serum intact parathyroid hormone and ionized calcium levels. With one exception, adynamic bone did not appear to be associated with lower bone density than other types of osteodystrophy, but a longer-term study is required to determine the complete natural history of this les