The post-transplant cytogenetic data were reviewed for 191 recipients of sex-mismatched marrow after conditioning with chemoradiotherapy as treatment for acute nonlymphoblastic leukemia. Host metaphases were detected transiently in unstimulated marrow from 12/184 patients and in PHA-stimulated peripheral blood from 18/140 patients during the first 100 days after transplantation. Of the 14 patients with HM who survived more than 150 days, five have relapsed, and nine are alive and in remission 509-1783 days after detection of HM. There was no significant correlation between transient detection of HM and pretransplant remission status, HLA matching, radiation regimen or graft-vs-host disease. We conclude that transient detection of HM by conventional cytogenetics is not related to any single peritransplant parameter and does not predict relapse.