Hemodialysis patients' compliance with their therapeutic regimens, including dietary and fluid restrictions, dialysis treatments and medications, is generally suboptimal. Recently, the mean age of the dialysis population has increased. Since impaired cognitive function, which sometimes accompanies aging, interferes with the ability to comprehend instructions, elderly dialysis patients might be at greater risk for noncompliance than are their younger counterparts. In this project, 135 hemodialysis patients (68 patients > 65 years of age and 67 patients < or = 65 years) were studied. Rates of noncompliance with oral antihypertensives and phosphate binders, as tracked by a medication event monitoring system (MEMS), are reported here. More than 42% of older patients (> 65) and 47% of younger patients (< or = 65) were repeated noncompliers with antihypertensives, missing at least 20% of the prescribed doses. Similarly, 65% of older patients and 80% of younger patients exhibited repeated noncompliance with phosphate binders. Almost 30% of older patients and more than 32% of younger patients missed their antihypertensives completely on 20% or more of the prescribed days. Only 18% of the older subjects, but 33% of younger patients missed taking their phosphate binders for 20 or more percent of the prescribed days. Rates of noncompliance were found to differ between the two populations of patients in that younger patients made significantly more dosing errors with their antihypertensives and missed taking their phosphate binders on more days than did their older counterparts.