Type IV nuclear bodies are classified as true intranuclear inclusions that ultrastructurally contain numerous densely packed 20- to 30-nm osmiophilic granules surrounded by a microfibrillar cortex. In the present study, we have found statistically significant ultrastructural differences in the frequency and size of type IV nuclear bodies in the in vitro bone marrow fibroblastic cells (FC) derived from eight nonleukemic subjects and 13 patients with acute nonlymphoblastic leukemia (ANLL) and myelodysplastic disorders (MDD). Patients with ANLL, MDD, and myelofibrosis, as a group, had four times as many type IV nuclear bodies as nonleukemic subjects. The mean frequency of type IV nuclear bodies for patients with ANLL and MDD was 2.68% +/- 3.27% as compared with 0.63% +/- 1.06% for the nonleukemic subjects (p less than 0.05). The mean maximum type IV nuclear body area of the ANLL and chronic myelomonocytic leukemia (CMML) patients as a group was 2.08 +/- 1.10 micron2, compared with a mean maximum area of 0.93 +/- 0.10 micron2 from nonleukemic subjects (p less than 0.05). The FC were otherwise morphologically indistinguishable and displayed the typical ultrastructural features of fibroblasts. These findings have provided the first morphologic evidence that supports the concept of an altered bone marrow microenvironment in patients with ANLL and MDD. Since type IV nuclear bodies are found in high frequency in virally infected tissues, our quantitative ultrastructural findings raise the possibility of a local viral infection that affects the bone marrow microenvironment of patients with ANLL and some MDD disorders.