The c-mos and c-myc proto-oncogenes have been assigned to bands q22 and q24, respectively, of human chromosome No. 8. A gain of chromosome No. 8 is the most common abnormality observed in myeloproliferative diseases. By using probes specific for the c-mos and c-myc genes, we have analysed the genomic DNA from peripheral blood and bone marrow samples from 15 patients with various malignant myeloid diseases, including leukemia and myelodysplasia, and from one patient with non-Hodgkin's lymphoma, all of whom have trisomy for chromosome No. 8. Except for one patient, the c-mos and c-myc genes were found in restriction fragments of germline size. In one patient with myelodysplasia, one c-myc allele was rearranged in a Hind III fragment, the other allele being normal. Thus, trisomy 8 associated with human hematologic neoplasia is generally not related to gross rearrangements of the c-mos or c-myc genes.