Amniocentesis was performed in 25 patients being treated with tocolytic therapy for premature labor, in an effort to determine the incidence of patients with mature lecithin/sphingomyelin (L/S) ratios and to discover a possible effect of tocolytic therapy on pulmonary maturity studies. Optical density measurements at 650 nm (OD650) and L/S ratio determinations were performed on the amniotic fluid. Repeat amniocentesis was performed in eight patients from 48 hours to 7 days after initiation of tocolytic therapy. There was a poor correlation between OD650 values of 0.150 or greater and L/S ratios of 2 : 1 or greater (64.3% false positives); and in the eight patients who underwent repeat amniocentesis, there was no significant change in the L/S ratio.