To define the influence that dialytic modality has on the blood pressure (BP) level and pattern, 33 hemodialysis (HD) and 27 peritoneal dialysis (PD) patients had their BP monitored hourly over an approximate 48-hour period using an ambulatory blood pressure monitoring (ABPM) device. A trigonometric cosine model was used to describe the diurnal BP pattern. Regression coefficients obtained from fitting this model to the observed hourly blood pressures were then compared between HD and PD patients to determine if the dialytic modality had any influence on BP level or pattern. The results indicate that HD and PD patients both exhibit similar diurnal patterns, but that HD patients have significantly higher average systolic BPs (142.1 +/- 16.3 v 130.4 +/- 17.1 mmHg, P < 0.01) and "systolic loads" (percent systolic values > 140 mmHg [54% +/- 29% v 30% +/- 31%, P < 0.01]) compared with PD patients. There were no significant differences in their diastolic BPs, diastolic loads, mean arterial pressures, or heart rates. No other factors (demographic or biochemical data, or medication usage) were found to significantly affect BP. In addition, a single BP reading for PD patients and predialysis and postdialysis BP readings for HD patients were measured by the dialysis nurse or technician on the day that the ABPM device was attached and removed, and were compared with the mean BP readings as determined by ABPM. These single values did not achieve good concordance with the 24-hour average BPs. ABPM and the cosine model have demonstrated that the diurnal pattern of BP is maintained in both PD and HD, and that HD is associated with higher systolic BPs and greater systolic loads than PD.