The first real-time, online measurements obtained using a screen-printed planar sensor to continually monitor urea in the spent dialysate are described. Evaluation of the sensor performance in the real clinical situation involves an extensive comparison of pre and post dialysis characteristics. Steady-state response and calibration curves show excellent reproducibility (typically within 5%; maximum deviation, 15%). The real-time results show that the characteristics predicted from the predialysis characterization are preserved during and after the prolonged period of continuous contact with the dialysate without needing recalibration. An operational stability of more than 10 hours of continuous use has been observed. Stability including intermittent wet-storage was found to be more than 5 days. Zero-baseline stability greater than 0.2 mmol/l (1.2 Og/dl) during the 4 hr treatment period and a resolution of 0.2 mmol/l were achieved, with a full-scale range of 15 mmol/l (90 mg/dl). A response time of less than 3 minutes allows dialysate urea levels to be monitored as a function of dialysis parameters such as varying blood flow. Parallel analysis of dialysate samples taken at 15 min intervals shows good correlation with the recorded data, although correspondence of the absolute values requires that the real-time data be rescaled. Because no degradation of the sensor performance was observed in vitro, this deviation can be explained only in terms of a dialysate based interference, probably caused by changes in pH and buffer.