Concentration of the 13,14-dihydro,15 keto-metabolite of prostaglandin F2 alpha (PGFM) was measured in women being observed for preterm labor. The mean initial PGFM level was significantly higher in patients who delivered preterm (65.9 +/- 9.7 pg/mL; N = 14) than in patients not in preterm labor (32.1 +/- 4.3 pg/mL; N = 11; P less than .01). Plasma PGFM concentrations decreased significantly during ritodrine therapy only in successfully treated patients (P less than .05). All patients with initial PGFM concentrations greater than or equal to 55 pg/mL delivered preterm. Two of four patients not considered to be in preterm labor but who delivered prematurely (within five days of initial evaluation) had initial PGFM concentrations of greater than 55 pg/mL. Concentration determinations of PGFM might be a useful adjunct in identifying early preterm labor and in predicting success of tocolytic therapy.