Acquired pseudo-Pelger-Huet neutrophils appeared in the peripheral blood of 11 of 83 Philadelphia-positive chronic myeloid leukemia patients during the blastic phase of the disease. Chromosomal analysis performed at the time of pseudo-Pelger appearance showed karyotype evolution with involvement of the short arms of a chromosome #17. In eight cases an i(17q) was present, in two cases an unbalanced translocation, and in one case monosomy. All these rearrangements had in common the loss of the distal end of the short arm. The morphologic and chromosomal study of the remaining 72 chronic myeloid leukemia patients demonstrated neither pseudo-Pelger nor 17p involvement.