We review our collective experience over the past 4 years using a portable instrument that measures the International Normalized Ratio (INR) and the prothrombin time (PT). The device can be used readily in ambulatory clinic settings and by patients in their homes. The major advantage of the monitor is that the INR/PT can be measured quickly; if a change in the dose of warfarin is necessary, instructions can be discussed directly with the patient at the time of the visit. Compared with clinical laboratory determinations, monitor INRs are quite accurate, with approximately 85% of measurements within 0.9 INR units of the corresponding laboratory value. If necessary, monitor INRs can be adjusted to approximate a specific clinical laboratory's INRs with a regression formula derived from analysis of duplicate INRs using the monitor and the laboratory. Although the Food and Drug Administration has not yet approved use of the monitor by patients at home, use of a portable measuring device in the home by home health agencies may be valuable in selected cases.