Cytogenetic analysis was successful in seven of eight sacral chordomas. Clonal chromosome aberrations were detected in three. One had a t(1;6)(q44;q11) as the sole aberration, one displayed three near-diploid clones that shared del(2)(p21), del(9)(p13), -10, structural rearrangements of 12q13, and add(19)(p13), and the third chordoma had two aberrant cell populations--one hypodiploid with several numerical and structural changes, the other hypertriploid-hypotetraploid with all marker chromosomes in duplicate. Including the present cases, seven cytogenetically aberrant chordomas have been reported. Five have had hypodiploid clones and the most frequent changes have been -3, -4, -10, and -13. No recurrent structural rearrangement has been identified; the only chromosome bands involved more than once are 1q21, 3q11, 5p15, 20q13, and 21q22.