The cytogenetic findings in a five-week-old female infant with acute lymphoblastic leukemia (ALL) are reported. Markers 11q - and 19 + were observed and considered to be due to an interstitial deletion of segment 11q13 to 11q23 of chromosome #11 and an insertion of this segment into the terminal region of the short arm of #19. Previously published banded cases of leukemic infants under one year of age have been summarized. A review of the data in these 29 cases suggests that the appearance of a normal karyotype in acute leukemia of infants (less than or equal to 1 year old) is much less common than in other categories of acute leukemia. Fourteen out of 29 cases (48%) had chromosomal abnormalities involving 11q. Seven of eight ALL cases had aberrations with a breakpoint at 11q22-23; six cases had t(4;11), one case had a del(11q) and ins(19p), and another had a t(1;22;4). All of three AMMoL cases had translocations involving the long arm of #11. The percentage of patients with t(4;11) and certain translocations involving 11q in infants with ALL or AMMoL, respectively, is higher than that seen in ALL and AMMoL in general. Eleven out of 12 cases (92%) of infant acute leukemias with chromosomal abnormalities involving 11q22-23 were five months old or less.