Calibration, i.e. the transformation in real time of the signal I(t) generated by the glucose sensor at time t into an estimation of glucose concentration G(t), represents a key issue for the development of a continuous glucose monitoring system. Objective: To compare two calibration procedures. In the one-point calibration, which assumes that I o is negligible, S is simply determined as the ratio I/G, and G(t)=I(t)/S. The two-point calibration consists in the determination of a sensor sensitivity S and of a background current I o by plotting two values of the sensor signal versus the concomitant blood glucose concentrations. The subsequent estimation of G(t) is given by G(t)=(I(t)-I o )/S. Research design and methods: A glucose sensor was implanted in the abdominal subcutaneous tissue of nine type 1 diabetic patients during 3 (n=2) and 7 days (n=7). The one-point calibration was performed a posteriori either once per day before breakfast, or twice per day before breakfast and dinner, or three times per day before each meal. The two-point calibration was performed each morning during breakfast. Results: The percentages of points present in zones A and B of the Clarke Error Grid were significantly higher when the system was calibrated using the one-point calibration. Use of two one-point calibrations per day before meals was virtually as accurate as three one-point calibrations. Conclusion: This study demonstrates the feasibility of a simple method for calibrating a continuous glucose monitoring system.