We performed fluorescence in situ hybridization (FISH) on bone marrow or peripheral blood cells thought to contain a del(12p) or an unbalanced 12p11-12 translocation from 17 patients who had various hematologic malignant diseases. We used 11 cosmid, phage, and plasmid probes which we had previously ordered on 12p. Cells from three patients with myeloid disorders were shown to have 12p13 translocations that involved chromosome 2 in two of them. Moreover, in all patients, FISH showed that the translocations were associated with proximal interstitial deletions which contributed to the difficulty in identifying these translocations. Our data suggest that some rearrangements of 12p which have been described previously as deletions or unbalanced translocations may, in fact, represent 12p13 translocations accompanied by an interstitial deletion.