Fifteen patients with hematologic disorders showed abnormalities involving chromosome band 13q14. Nine patients had an interstitial deletion of this band, similar to that reported in some retinoblastoma tumors and as a constitutional abnormality in a small proportion of cases of familial retinoblastoma. In five patients, band 13q14 was involved in translocations and in one case there was a deletion of one chromosome #13 and a translocation involving the homologous #13. The diagnosis in the majority of our patients (11 of 15) was chronic lymphocytic leukemia. In these patients the abnormalities were detected in cultures stimulated with 4-phorbol 12-myristate 13-acetate (PMA). It is possible that the utilization of this agent is a fundamental requirement for the reliable demonstration of abnormalities involving 13q14 in patients with B-cell malignancies. The incidence of abnormalities involving 13q14 and their significance in the development of neoplasias, other than retino-blastoma, is discussed.