Here we describe in detail both the expression of Hoxa-11 in the wild-type mouse uterus and the defects resulting in maternal reproductive failure of Hoxa-ll null female mice. The Hoxa-11 gene is expressed at peak levels in uterine stromal cells during metestrus. Hoxa-11 transcripts were induced beginning on Day 2 of gestation in the stromal cells underlying the uterine epithelium and appeared in the secondary decidual zone between Days 6 and 8 of gestation. At early gestational stages, stromal, decidual, and glandular cell development were deficient in Hoxa-ll null uteri in comparison to wild-type as assessed by histology and immunohistochemical localization of the decidual cell marker epitope, stage-specific embryonic antigen- 3 (SSEA-3). Both steroid-induced uterine stromal and glandular cell proliferation as well as oil-induced stromal decidualization after induction of pseudopregnancy were deficient in mutant uteri. Moreover, both Western blotting and immunohistochemistry demonstrated that the burst of glandular leukemia inhibitory factor (LIF) found in normal pregnant uteri at Day 4.5 of gestation was absent in Hoxa-11-deficient uteri. The LIF burst was also not observed in the uteri of bilaterally ovariectomized, hormonally stimulated Hoxa-11 mutants. These results demonstrate that the Hoxa-l1 gene is required for normal uterine stromal cell and glandular differentiation during pregnancy, as is the presence of the steroid-induced glandular LIF burst initiating embryo implantation.