<jats:title>Abstract</jats:title><jats:sec><jats:title>Objectives</jats:title><jats:p>Post‐void residual urine volume (PVR) and bladder voiding efficiency (BVE) are widely used as clinical parameters to evaluate patients with voiding dysfunction. The present study was conducted to assess the variability of PVR and BVE determinations in patients with underactive bladder (UAB). In addition, we focused on the bladder volume prior to voiding (BV<jats:sub>void</jats:sub>) that may influence PVR and BVE, and investigated a correlation between PVR and BV<jats:sub>void</jats:sub>, and between BVE and BV<jats:sub>void</jats:sub>.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>Ten patients with a symptom complex of UAB, who had PVR of 50 mL or greater, were admitted to hospital during a 24‐hour period for the measurement of voided volume (VV) and PVR. PVR was measured by transabdominal ultrasonography. BVE was expressed by a fraction (%) of bladder volume evacuated ([VV/BV<jats:sub>void</jats:sub>] × 100).</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>Ten patients, five men (mean age of 65.0 years) and five women (mean age of 70.2 years), participated in this study. Regardless of gender, there was a large variation in repeated measurements of PVR in an individual patient. PVR increased with an increase in BV<jats:sub>void</jats:sub>, and there was a significant linear relationship between PVR and BV<jats:sub>void</jats:sub>. BVE was approximately constant after every voiding in each patient, and there was no significant linear relationship between BVE and BV<jats:sub>void</jats:sub>.</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>Measurement of PVR was unreliable because of wide variation in the same individual. The variation of BVE was much smaller than PVR. BVE would be a reliable parameter with good reproducibility for the assessment of emptying function.</jats:p></jats:sec>