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We have developed a system to perform fully ambulatory monitoring studies of the bladder. Bladder and rectal pressures are recorded during natural filling of the bladder for up to 6 hours. This system has been evaluated in 19 studies performed on 15 patients. The mean duration of the studies was 3 hours 51 minutes (standard deviation +/- 1 hour 3 minutes) and the mean number of voids per patient was 3.8 +/- 2.2. Visual inspection of the recorded data showed that subtraction of the rectal trace from the bladder trace provided a useful detrusor trace when subjects were ambulant as well as resting. When ambulant, movement caused typical peak-to-peak pressure variability in the bladder and rectal traces of approximately 30 +/- 20 cm. water (mean and standard deviation of peak-to-peak pressure for typical ambulant 5-minute sections for each patient), which after subtraction was reduced on the detrusor trace to 10 +/- 5 cm. water. The equivalent figures when resting were 9 +/- 5 cm. water, subtracting to less than 5 cm. water for the detrusor. Further measurements also were calculated for the whole of each trace. The system provided good quality recordings and should prove useful in future evaluations of natural fill urodynamic studies.