To quantify the potential cost savings if azithromycin is substituted for erythromycin in women with preterm premature rupture of membranes (PPROM). Secondary analysis of a multicentered study investigating magnesium sulfate for the prevention of cerebral palsy in premature infants. All patients with PPROM who received antibiotics for prophylaxis were included in the analysis. The number of expected doses each patient would have received was calculated for erythromycin, multidose azithromycin, and single-dose azithromycin regimens accounting for latency from PPROM to delivery. The wholesale acquisition cost was used to calculate the expected cost of each regimen. There were 981 PPROM patients who received a penicillin class antibiotic and erythromycin. Patients would have received 7,528 intravenous doses and 10,194 oral doses of erythromycin at a combined cost of $357,169. In comparison, patients would have received 6,422 and 3,942 doses at a cost of $15,669 and $9,574 for the multidose and single-dose azithromycin regimens respectively, which represents a more than 95% cost reduction for either regimen compared with erythromycin. The use of azithromycin substituted for erythromycin in the standard antibiotic regimen of women with PPROM represents a potential for substantial cost reduction.