To assess the expectations of preterm birth prevention, we determined the causes of preterm birth in a population of indigent women. We studied 13,119 singleton births in a predominantly black, indigent population occurring between November 1982 and April 1986 to identify the proportion of preterm births that may have been prevented using current treatment modalities. Forty-four percent of the preterm births occurred at 35 to 36 weeks' gestational age, a time when most practitioners do not attempt tocolysis. Of the remainder, 17% occurred before 35 weeks but were indicated for maternal medical or obstetric complications, and another 17% occurred before 35 weeks but followed spontaneous premature rupture of the membranes. Therefore, of the 1445 preterm births, we calculated that only 336 (23.2%) were theoretically preventable. A fourth of these presented at less than 3 cm cervical dilatation and were treated appropriately with tocolytics, but delivered anyway. Therefore, most of the potentially preventable births occurred in the group that presented with cervical dilatation of more than 3 cm. We conclude that improving the preterm birth rate significantly below current levels may be difficult to achieve.