Dyserythropoiesis after bone marrow transplantation is common, but the presence of ringed sideroblasts (RS) has not been evaluated fully. To examine for RS, a combined silver and Perls' Prussian blue stain, shown previously to be more sensitive for detecting RS than Perls' Prussian blue stain alone, was used on post-transplant marrow aspirate sections from 39 patients who received marrow transplants (allogeneic, 28; autologous, 11) for a variety of disorders. Marrow aspirate sections were available for comparison from 11 of these patients before any treatment as well as from five patients with normal marrows and normal peripheral blood cell counts. Aspirates were not performed on donor marrows. By the modified silver stain, RS were present in 34 (87%) patients whose marrows were sampled 0.5 to 39 (median, 1.5) months post-transplant including 10 of 11 patients with autologous transplants (no graft versus host disease prophylaxis). In contrast, seven of 36 (19%) of these marrows contained RS when stained with Perls' reaction alone. Only one of 11 pretransplant marrows and none of five normal marrows contained RS when stained by either method. These results demonstrate that RS are present in most post-transplant marrows even beyond the usual period of reconstitution (28 days), and this finding can be included among the features of dyserythropoiesis seen after transplantation. RS apparently are not related to pretransplant pathology or post-transplant therapy. This study also confirms previous observations that modified silver stains are more sensitive for detecting RS than Perls' Prussian blue stain.