To evaluate the effect of autologous endometrial coculture in patients (less than 36 years old) with a history of a single IVF failed cycle associated with poor quality embryos. Controlled clinical study. University-based in vitro fertilization center. Twenty-six patients with a history of a single prior failed IVF-ET with poor preembryo quality. Autologous endometrial coculture. Preembryo blastomere numbers and cytoplasmic fragmentation rates were compared between the treatment and previous cycle. Clinical pregnancy rates were analyzed. Twenty-six women with an average age of 32.8 +/- 2.9 years underwent treatment. On Day 3 the overall mean number of blastomeres per preembryo on coculture compared to conventional medium in a previous cycle was 6.1 +/- 1.8 vs. 5.1 +/- 1.3 (P = 0.01; Wilcoxon test). The average percentage of cytoplasmic fragments on coculture compared to the conventional medium in a previous cycle was 14% +/- 10 vs. 22% +/- 13 (P = 0.003; Wilcoxon test). At transfer the mean number of blastomeres per preembryo on coculture was 7.4 +/- 1.8 compared to 6.7 +/- 1.5 on conventional medium in a previous cycle (P = 0.02; Wilcoxon test). The clinical pregnancy rate (positive fetal cardiac activity) per patient was 88.5%. The delivery rate was 73.1% (19/26). There was an improvement in the preembryo quality for preembryos on autologous endometrial coculture compared to noncocultured preembryos from the same patient in a previous cycle. An excellent delivery rate was subsequently found.