Mortality rates among American hemodialysis patients are the highest in the industrialized world. Measures of delivered dialysis (Kt/V) correspond strongly with survival and are estimated to be inadequate in one third of patients. We sought to determine the importance of potential barriers to adequate dialysis, including patient-related and technical factors. Using a cross-sectional study design, we abstracted the charts of 721 randomly selected patients from all 22 chronic hemodialysis units in northeast Ohio. For each of 1,836 treatments provided to these patients, we assessed delivered dialysis (Kt/V) and patient-related factors (ie, hypotension, intradialytic symptoms, and treatment time missed due to noncompliance or transportation problems) and technical factors (ie, dialysis prescription, type of vascular access, clotting, and dialyzer reuse). We used hierarchical regression analysis to determine which potential barriers were independently related to delivered dialysis after adjustment for patient demographic and medical characteristics. Barriers independently related to dialysis delivery (all P values < 0.001) included patient noncompliance, present in 3% of treatments; low dialysis prescription, 14%; use of a catheter for vascular access, 11%; and clotting, 1%. The prevalence of identified barriers varied dramatically across facilities (eg, the prevalence of low dialysis prescription ranged from 0% to 37%, while the prevalence of catheter use ranged from 3% to 28%). In conclusion, patient noncompliance, low dialysis prescription, catheter use, and clotting are the most important barriers to dialysis delivery. Further work is needed to develop interventions to overcome these barriers and to determine the effect of such interventions on dialysis adequacy and patient survival.