This retrospective study compared the fetal lung maturity biochemical profile of patients having preeclampsia with that of patients having preterm labor. Study Design: Amniotic fluid was obtained by transabdominal amniocentesis in 90 patients, 59 patients with preterm labor (PTL) and 31 patients with preeclampsia (PRE). Pregnancies with fetal growth restriction were excluded. Fetal lung maturity was assessed by lecithin/sphingomyelin ratio (L/S) and by a fluorescence polarimetry assay (FLM). Mean values of WS ratios and FLM were compared between the PTL and the PRE groups, each within two gestational age subgroups (27-32.9 weeks gestation and 33-36 weeks gestation). Student t-test, Chi-square test Fisher's exact test were used for statistical analysis. A p value < 0.05 was considered significant. Results: During the gestational age interval of 33-36 weeks, the mean LIS ratios were significantly lower in pregnancies complicated by PRE than in those complicated by PTL (1.99 +/- 0.26 and 2.4 +/- 0.57, respectively; p = 0.01). Similarly, during this gestational age interval, the FLM values were also lower in PRE than in PTL, although the difference did not reach statistical significance. Conclusion: During the gestational age between 33 and 36 weeks of gestation, the biochemical profile of preeclamptic patients without IUGR has a significant lower L/S ratio compared to that of preterm patients.