Erythromycin is the recommended therapy for pregnant women with chlamydial infection. Despite the fact that this drug has been available since 1952, relatively little is known about its pharmacokinetic behavior in pregnant women, and no investigations have been conducted in women who were in the third trimester of pregnancy. In this study, 10 women were treated with erythromycin for chlamydial infection during pregnancy; 7 of these women were in the third trimester of pregnancy, and 3 were in the second trimester. Serum samples were obtained at 0.5, 1, 2, 3, and 4 hours after a 500-mg oral dose of erythromycin base and were analyzed to determine absorption and peak serum levels of erythromycin. Results indicated that absorption was delayed and serum levels were diminished in comparison with values reported in the literature for patients in the second trimester of pregnancy. In 2 women, erythromycin serum levels were not detectable at any time during the 4 hours of the study; these 2 women also experienced the most severe gastrointestinal symptoms. Although the size of our study population was small and certainly not definitive, the data suggest that in patients in the third trimester of pregnancy, severe adverse gastrointestinal events may forewarn of subtherapeutic plasma concentrations of erythromycin, which could have consequences for the treatment outcome.