The purpose of this study was to assess the effects of maternal smoking on amniotic fluid volume and fetal urine output. Seventeen women (10 smokers and 7 non-smokers) were serially assessed through pregnancy. Smoking status was determined by self-report and carbon monoxide manometry. Amniotic fluid volume was assessed by sonographically measuring the maximum vertical pocket, the amniotic fluid index, and the two dimensions of the largest pockets. The coronal area formula was used to calculate bladder volume at 5-minute intervals for 20 minutes. Urine output was calculated from serial measurements of bladder volume. The mean gestational age of the first session was 26.8 (2.7) weeks, of the second session 32.4 (1.6) weeks, and of the last session 36.9 (2.1) weeks. In the univariate analysis there were no differences between smokers and non-smokers at any of the three visits for gestational age, fetal weight, maximum vertical pocket, amniotic fluid index, two-dimensional maximum pocket, and urine output. In the regression model, while controlling for gestational age, smoking status could not predict significant differences in any parameter. Maternal smoking does not appear to affect either amniotic fluid volume or fetal urine output.