Wired Enzyme (Therasense, Alameda, CA) sensing technology for continuous measurement of in vivo glucose concentrations offers the benefits of (1). excellent sensor stability, (2). reduced susceptibility to variations of in vivo oxygen concentration, and (3). minimized response to common electroactive interferents. This study describes the response of 48 sensors (25 in the upper arm, 23 in the abdomen) implanted for 3 days in patients with Type 1 diabetes. Prospective calibration was performed using capillary blood; results were compared with venous plasma glucose values obtained at 15-min intervals. Ninety-eight percent of readings fell in the clinically accurate Clarke error grid zone A or clinically acceptable zone B. Choice of the site of the implanted sensor (upper arm vs. abdomen) or the capillary blood calibration site (arm vs. finger) did not affect system accuracy.