Flow cytometric DNA analysis was performed on 203 paraffin-embedded archival specimens obtained from patients with surgical stage I endometrial carcinoma. Primary therapy for those patients (1979-1983) had been definitive extirpation with adjuvant therapy determined by histologic grade, histologic subtype, myometrial invasion, and peritoneal cytologic findings. Diploid DNA patterns were identified in 171 (84%) specimens and nondiploid characteristics were observed in the remaining 32 (25 DNA aneuploid, 7 DNA tetraploid). Although DNA nondiploid specimens accounted for only 16% of all stage I patients, they accounted for 50% of all relapses. Regardless of treatment or other pathologic features, progression-free 5-year Kaplan-Meier survival estimates were 92 and 63% for patients with DNA diploid and DNA non-diploid patterns, respectively (P less than 0.001). Overall 5-year progression-free survival for patients with grade 1 or 2 lesions was 90%; stratification by DNA diploid and DNA nondiploid patterns revealed progression-free survivals of 94 and 64%, respectively (P less than 0.001). Peritoneal cytologic study was positive in seven patients; none of the five with a DNA diploid pattern had a relapse and both with the DNA nondiploid pattern had relapses. These studies suggest that DNA ploidy status may be an objective prognostic determinant for patients with stage I endometrial carcinoma.