The incidence, type, and clonality of isochromosomes at diagnosis were investigated in acute lymphoblastic leukaemia (ALL). An isochromosome was detected in 50/1,035 (4.8%) of successfully karyotyped patients, 41/919 children (4.5%) and 9/116 adults (7.8%), who were diagnosed within a 5 year period. Isochromosomes of 21q with breakpoints in the short arm at p11 or in the long arm at q10 or q22 were identified in 15 patients (1.4%) associated with B-lineage immunophenotype, a white blood cell count (WBC) of < 10 x 10(9)/litre, and pseudo- or low hyperdiploidy. Isochromosomes of 17q and 7q occurred in 13 (1.3%) and 9 (0.9%) patients, respectively, and were associated with high hyperdiploidy. Isochromosomes of 9q and 6p occurred in 6 (0.6%) and 5 (0.5%) patients, respectively, whereas i(Xp), i(lq), and i(8q) occurred in I patient each. The isochromosome occurred as the sole abnormality in 4 patients [3 with i(21q) and I with i(7q)] and in the stemline, but with other chromosomal changes, in 35 patients. It was confined to a clonally evolved sideline in II patients. Isochromosomes occurred with established abnormalities in 7 patients: with t(1;19)-i(7q)/i(9q)/i(7q) and i(9q) each in I patient; with t(4;11)-i(7q)/i(17q) in 1 and 2 patients, respectively; and with t(9;22)-i(9q) in I patient. This study indicates that isochromosome formation can be an early chromosomal change and suggests that i(21q) occurs more frequently at diagnosis than has been previously suspected.