beta(2)-microglobulin (beta(2)-M) amyloidosis is an important cause of morbidity in patients on dialysis. In this cross-sectional study, we evaluated supraspinatus tendon thickness (as a measure of shoulder involvement from beta(2)-M amyloidosis) in patients who are on hemodialysis (HD) compared with those on continuous ambulatory peritoneal dialysis (CAPD). In 27 patients on HD who were treated with high-flux dialyzers, 31 patients on CAPD, and 31 healthy volunteers, we performed bilateral shoulder magnetic resonance imaging and measured the supraspinatus tendon thickness using electronic calipers. There were no statistically significant differences in age or dialysis duration between the HD and CAPD patients. Each patient was asked about the presence or absence of shoulder pain. The supraspinatus tendon thickness in HD patients (mean thickness 6.6 +/- 1.3 mm, range 3.20-8.80 mm, N = 53) and CAPD patients (6.8 +/- 0.9 mm, range 4.9-8.8 mm, N = 61) was not significantly different (P = 0.289); however, the mean thickness in either group was higher than in the healthy controls (5.5 +/- 0.6 mm, range 4.3-6.8 mm, N = 61) (HD patients vs. controls: P = 0.000; CAPD patients vs. controls: P = 0.000). Patients with shoulder pain had higher mean supraspinatus tendon thickness measurements than patients without shoulder pain (P = 0.042). The thickness of supraspinatus tendons is not significantly different between patients on CAPD and HD. An association exists between shoulder pain and mean supraspinatus tendon thickness. This hidden complication of ESRD should be further studied in larger populations of dialysis patients.