We analyzed the effect of a 24-hour total food and water fast on the spontaneous onset of labor by studying the effect of the fast of Yom Kippur on Jewish pregnant women in Jerusalem. The mean baseline number of deliveries for our study period was 11.9 and 12.0 for 1981 and 1982, respectively. The number of deliveries on Yom Kippur was not significantly different from the mean. However, the number of deliveries for the 24-hour period after termination of the fast was 22 and 26 for 1981 and 1982, respectively. This difference attained statistical significance. While the clinical implications are not yet clear, a special risk may exist for women with a tendency toward early delivery should they undertake a 24-hour fast.