Bladder outlet obstruction (BOO) with benign prostatic hyperplasia (BPH) was determined by pressure-flow studies. Residual urine volume was measured by urethral catheterization. Fifty-one patients with or without BOO were investigated for residual urine. The mean residual urine volume was 32.8 +/- 35.9 ml and 25.2 +/- 22.7 ml in BOO group (31 patients) and no BOO group (20) respectively. The range of residual urine volume was from 6 to 210 ml and from 3 to 88 ml in the two groups respectively. Residual urine volume was not statistically different between the BOO group and no BOO group (P > 0.05). The results of pressure-flow studies were significantly different between the BOO group and no BOO group (P < 0.01). Residual urine determination was not a reliable criterion for selection of patients for surgery. The cause of residual urine is not only BOO. The patients with BOO may present insignificant residual urine. It is evident that the absence of post-void residual urine does not rule out BOO.