The following rare Ph'-positive chromosome constitutions, based on the cyto-genetic findings in three cases with acute leukemia, are presented. 1) A hypo-diploid karyotype! primarily 43,-X,-7,-8,!3p+ and a Ph', in a patient with acute lymphoblastic leukemia (ALL) in relapse, followed by a complete remission and a normal chromosomal picture and then by the appearance of cells with a 46,XX,Ph' karyotype. The Ph' was due to a standard translocation between chromosomes #9 and #22. 2) The first demonstration of an unusual Ph'-translocation between chromosomes #19 and #22 in a condition other than chronic myelocytic leukemia (CML), i.e., acute myeloblastic leukemia (AML). 3) The presence of a Ph' in acute erythroleukemia (EL) due to a translocation between chromosomes #4 and #22, this apparently being the first description of such a translocation in any disease. The cytogenetic findings, particularly those in the Ph'-positive case of ALL, were evaluated in relation to the cytologic and immunologic features, clinical courses and implications, and the interrelationship between the three conditions (AML, blastic phase of CML and ALL), which have to be considered in cases of Ph'-positive acute leukemia.