A series of 20 patients underwent conventional medium fill cystometry (CMG) and ambulatory monitoring during natural bladder filling (AM). The measurement of voiding pressures by the 2 techniques was compared. The maximum subtracted detrusor contraction pressure recorded during CMG (50 +/- 30 cm H2O) was significantly less than that recorded during AM (86 +/- 35 cm H2O). Voiding pressures during natural filling are greater than those observed during conventional urodynamic studies: this finding may have important implications in the definition of bladder outflow obstruction.