Malignant cells from 24 cases of hairy cell leukemia were studied by in situ hybridization for evidence of selective aneuploidy using alphoid and satellite probes specific for 16 human chromosomes. Based on these data, hairy cell leukemia appears to be diploid for the chromosomes studied and is a malignancy which displays the phenomenon of pairing of the centromere and p arm of chromosome 15 during interphase.