Transvaginal ultrasound has been presented as a technique for measuring bladder volumes in women in the range 2 to 175 ml, thus overcoming the limitations of abdominal ultrasound at these smaller, though clinically important volumes. The mean error of the technique in a preliminary study using known volumes from 10 to 175 ml was 23%. The present study used the unknown bladder volumes of 41 women undergoing gynaecological surgery who required initial urethral catheterisation. The bladder volumes predicted by transvaginal ultrasound were compared with the volumes obtained when the bladder was emptied by urethral catheterisation. The mean accuracy rate was 24% and the optimum range was 50 to 200 ml. The applications of this technique to the measurement of residual urine in clinical practice include the management of women with voiding difficulties and monitoring women who are either self-catheterising or who have indwelling suprapubic catheters.