The GlucoWatch Biographer (Cygnus, Inc., Redwood City, CA), a novel, noninvasive method for tracking glucose, makes frequent, automatic glucose measurements. Therefore, it potentially can provide a convenient method of tracking glycemia, particularly with exercise as an added variable in diabetes management. The purpose of the investigation was (1) to measure the effectiveness of this device, and (2) to determine the validity of its readings, during rest and exercise both indoors and outdoors. Four subjects with type 1 diabetes and five controls without diabetes participated in rest, mild [40% of heart rate reserve (HRR) for 45 min], moderate (60% of HRR for 30 min), and heavy exercise (80% of HRR for 15 min) both indoors and outdoors. Subjects monitored blood glucose levels (forearm alternate site testing), and readings were taken by the GlucoWatch during and following testing conditions. Significantly more GlucoWatch readings were obtained during rest (55.6% outdoors, 66.7% indoors) than during moderate (11.1%) or heavy (0%; P < 0.01) exercise. A similar number was obtained during mild outdoor exercise (46.6%), but not indoors (29.2%). Sweating rates corresponded to a reduced effectiveness during indoor exercise only (P < 0.01), where sweating was heavier despite higher outdoor temperatures. A weak association between GlucoWatch and matched glucometer readings (r = 0.81; P < 0.01) was found. The effectiveness of the GlucoWatch Biographer during any moderate to heavy exercise was extremely poor, as was the validity of its readings compared with forearm alternate site testing. Thus, anyone wishing to engage in daily exercise would be best advised to abstain from wearing the GlucoWatch except during rest or mild exercise.