To introduce the proper voiding modality to patients with myelodysplasty, urethral opening pressure (UOP), an intravesical pressure just at the beginning urine flows out beyond the external urethral sphincter, was measured in 63 myelodysplastic patients. Among 45 renal units with any morphological or functional changes at the first UOP measurement, 37 units (82.2%) were included in the high UOP group (> or = 35 cmH2O). And among 41 ureters with VUR of more than grade 2, 32 (78.0%) were in the high UOP group. In addition, deformity of the urinary bladder was observed in 36 patients, and 26 (72.2%) of these bladders showed high UOP values. Therefore, all the patients could be divided into two groups: high UOP group (> or = 35 cmH2O, 28 cases) and low UOP group (< 35 cmH2O, 35 cases). Twenty-three patients (82.1%) with high UOP values had been mainly treated with clean intermittent catheterization (CIC). In contrast, 24 patients (68.6%) with low UOP values had been allowed to urinate by Credè' or Valsalva's method. In the followup study for 40 to 44 months, patients in the CIC group obtained good prognosis as for morphological or functional changes of the urinary tract. On the other hand, patients in the Credè' or Valsalva's method group showed a significantly higher deterioration rate in the high UOP group (80.0%) than that in the low UOP group (9.1%) (p < 0.005). From these results, hopely that in myelodysplastic patients with the underactive detrusor, CIC may be introduced for low pressure voiding to those who show high UOP values as early as possible. On the other hand, those who show low UOP values may be managed with Credè' or Valsalva's method as well as CIC. Thus, UOP is considered a possible prognostic factor for the morphological and functional changes of the urinary tract, which may be a useful parameter in decision of voiding modalities in myelodysplastic patients.