We have developed a new telemedicine system that uses a cellular telephone and Internet Web site to monitor changes of blood pressure (BP) in patients on continuous ambulatory peritoneal dialysis (CAPD). An I-converter data collection system directly transmits all data on BP and heart rate (HR) measured by a fully automatic device (HEM-705IT: Omron Life Science, Tokyo, Japan) to a cellular telephone. The cellular telephone then sends the data directly to the main server at the NTT DoCoMo data center. All data, including BP and HR from each patient, are accumulated in the server. Application service provider technology (ASP) connects that system directly to the Internet. Any time and anywhere, each patient can use a cellular telephone or the Internet Web site to monitor, in graph form, changes in their data. The average data at each collection point are calculated and shown on the Web site. All data collected by cellular telephone are also calculated and sent directly, in real time, to the physician's office. Currently, we are using this telemedicine system for 44 hypertensive patients on CAPD at our center. In the present study, we used the telemedicine system to evaluate changes in 24-hour BP. We followed 20 CAPD patients, monitoring changes in BP for 3 months. At the end of study, we found that the average daytime home BP was 137 +/- 8 mmHg over 80 +/- 4 mmHg and that the average nighttime BP was 128 +/- 7 mmHg over 72 +/- 4 mmHg. The overall average home BP was 132 +/- 7 mmHg over 76 +/- 3 mmHg. The average outpatient clinic BP was 155 +/- 9 mmHg over 86 +/- 5 mmHg. The measured systolic and diastolic BP at the out-patient clinic were significantly higher than those measured at home. As measured at the outpatient clinic, BP indicated that 7 CAPD patients (35%) were hypertensive (> 160/95 mmHg). However, as measured at home, BP indicated that only 4 CAPD patients (20%) were truly hypertensive (> 135/85 mmHg). We were therefore able to determine that 43% of our hypertensive patients had "white coat" hypertension. Our new telemedicine system that monitors BP by cellular telephone provides great advantages in monitoring BP at home in patients on CAPD.