Deletion 7q is a common chromosomal abnormality in myeloid neoplasms. Detection of del(7q) in patients following cytotoxic therapies is highly suggestive of an emerging therapy-related myeloid neoplasm. In this study, we describe 39 patients who acquired del(7q) as a sole abnormality in their bone marrow following cytotoxic therapies for malignant neoplasms. The median interval from cytotoxic therapies to detection of del(7q) was 40 months (range, 4-190 months). Twenty-eight patients showed an interstitial and 11 showed a terminal 7q deletion. Fifteen patients (38%) had del(7q) as a large clone and 24 (62%) as a small clone. With a median followup of 21 months (range, 1-135 months), 18 (46%) patients developed therapy-related myeloid neoplasms, including all 15 patients with a large del(7q) clone and 3/24 (12.5%) with a small clone. Of the remaining 21 patients with a small del(7q) clone, 16 showed no evidence of therapy-related myeloid neoplasms and 5 had an inconclusive pathological diagnosis. We conclude that isolated del(7q) emerging in patients after cytotoxic therapy may not always be associated with therapy-related myeloid neoplasms in about half of patients. The clone size of del(7q) is critical; a large clone is almost always associated with therapy-related myeloid neoplasms, whereas a small clone can be a clinically indolent or transient finding. Abnormalities of chromosome 7, either monosomy (−7) or deletion of the long arm (del(7q)), are the most common cytogenetic abnormalities detected in acute myeloid leukemia and myelodysplastic syndromes. 1-3 Del(7q)/ − 7 occurs in~8% of de novo acute myeloid leukemia and~10% of de novo myelodysplastic syndromes. 1,4 Del(7q) has been classified as the intermediate-II risk group in de novo acute myeloid leukemia 5 and the intermediate risk group in de novo myelodysplastic syndromes, respectively. 6 Therapy-related myeloid neoplasm is a late complication of cytotoxic therapies for primary malignant and non-malignant diseases. Cytogenetic abnormalities can be detected in more than 90% of patients with therapy-related myeloid neoplasms, and more than half of these patients have a complex karyotype. 7 Del(7q)/ − 7 presents in~50% of patients with therapy-related myeloid neoplasms and are often associated with prior exposure to alkylating agents or ionizing radiation. 1,[8][9][10][11][12][13] Any newly emerged cytogenetic abnormalities in patients with a prior history of cytotoxic therapies often raises the concern of development of therapy-related myeloid neoplasms, especially when the abnormalities include del(7q)/ − 7.In our practice, we have observed that some patients develop isolated del(7q) in their bone marrow following cytotoxic therapies, yet never develop therapy-related myeloid neoplasms with close follow-up. In order to better understand the