Defects of 3q in bands q21 and q26 have been reported in more than 70 cases of acute nonlymphocytic leukemia (ANLL), myelodysplastic syndrome (MDS), and myeloproliferative disorder (MPD) in blast crisis. In this paper three additional patients are described: patient 1 with refractory anemia with excess of blasts in transformation (RAEB-T) and inv(3)(q21q26), patient 2 with RAEB-T and t(3;3)(q21;q26), and patient 3 with myelofibrosis with myeloid metaplasia (MMM) in blast crisis and inv(3)(q21q26). In addition to 3q rearrangements, monosomy 7 and del(7)(q22q36) were observed in patients 1 and 2, respectively. In the three patients, the most characteristic clinical features were elevated platelet counts, marked hyperplasia with dysplasia of the megakaryocytes, and poor prognosis. Although disturbance of thrombopoiesis was not systematically observed in all patients with t(3;3)(q21;q26), inv(3)(q21q26), and ins or dup(3)(q21----q26), study of the 77 cases reported and of the three cases presented here brings further evidence to the existence of a cytogenetic syndrome involving bands q21 and q26 simultaneously, which represents a subtype of ANLL, MDS, and MPD, characterized by normal or elevated platelet counts, hyperplasia with dysplasia of megakaryocytes, multilineage involvement, young median age of patients with MDS, preferential involvement of women in t(3;3), high incidence of chromosome 7 defects in MDS and ANLL, short duration of the MDS phase, no response to chemotherapy, short survival, and por prognosis.