Chromosome investigation in six localized forms of tenosynovial giant cell tumors, also known as modular tenosynovitis, revealed an identical translocation between chromosomes 1 and 2, t(1;2)(p11;q35-36) in three tumors, a variant translocation t(1;5)(p11;q22) in a fourth case, and a t(2;16)(q33;q24) in a fifth case. One case showed a normal karyotype. Although morphologically rather uniform, these benign tumors appear to be cytogenetically heterogeneous, but the chromosome changes seem to cluster in 2 regions, 1p11 and 16q24.