The relationship between the types of dialysis membrane used and the prevalence and severity of radiolucent bone cysts (which are a main radiological feature of dialysis amyloidosis) was studied in 30 patients on hemodialysis for more than 10 years. One of them was treated exclusively with cuprophane; the other 29 were dialyzed with cuprophane, and then treated with polyacrylonitrile AN 69. In 12 of the 30 patients, radiolucent bone cysts (at least 5 mm in diameter in the wrists and at least 10 mm in the shoulders or hips) were observed. The patients with bone cysts spent significantly more time on cuprophane dialysis and significantly less time on AN 69 dialysis than the group of patients without bone cysts. Nine of the 14 patients who had been treated with cuprophane for more than 8 years had bone cysts; whereas bone cysts were observed in only 2 of the 12 patients dialyzed for more than 8 years with AN 69. The frequency of bone cysts was significantly different for each of the two groups. There was, however, no significant difference in the total duration of dialysis between the two groups. The severity of the cystic bone lesions correlated positively with the duration of dialysis using cuprophane and negatively with the duration of dialysis using AN 69. These findings suggest that the development of osteoarticular amyloidosis may be related to the type of dialysis membrane used. Hemodialysis using AN 69 membranes may prevent, or at least postpone the development of dialysis amyloidosis.