Subclinical intrauterine infection is an important cause of preterm labor, specifically where tocolysis has failed. Fifty patients in preterm labor with singleton pregnancies were studied prospectively to determine whether the presence or absence of C-reactive protein, a nonspecific marker for infection, would correlate with success or failure of tocolysis. Of the 50 patients, tocolysis failed in 11 of 15 women with a positive C-reactive protein determination. Tocolysis succeeded in 33 of 35 cases where C-reactive protein was negative (P less than .0005). Urinary tract infection occurred in 40% of the study patients, but was not a confounding factor in the interpretation of C-reactive protein.