Objective: To evaluate the effect of autologous endometrial coculture versus conventional medium on preembryo development. Design: Controlled systematic clinical study. Setting: University-based IVF center. Patient(s): Women with a history of failed IVF-ET with poor preembryo quality. Intervention(s): Patients underwent a luteal phase endometrial biopsy. The tissue then was digested enzymatically, and the stromal and glandular cells were separated by differential sedimentation rates. These cells were cultured to confluence, released, and then cryopreserved until the patient's IVF-ET cycle. All normally fertilized oocytes then were allocated systematically to growth on autologous endometrial coculture or conventional medium until transfer on day 3. Main Outcome Measure(s): Preembryo blastomere numbers and cytoplasmic fragmentation rates were measured. Result(s): Forty-two women underwent 44 cycles of IVF-ET. In the morning on day 3, the mean (±SD) number of blastomeres and cytoplasmic fragments per preembryo on coculture compared with conventional medium was 5.9 ± 1.5 versus 5.5 ± 1.4 and 21% ± 13% versus 24% ± 11. At transfer the mean (±SD) number of blastomeres per preembryo on coculture was 7.4 ± 1.3 versus 6.7 ± 1.9 on conventional medium. Conclusion(s): There was a significant improvement in the mean (±SD) number of blastomeres per preembryo and decrease in the fragmentation rate for preembryos on autologous endometrial coculture compared with noncocultured preembryos from the same patient.