Acute promyelocytic leukemia (APL) is characterized by a consistent chromosomal aberration that fuses the retinoic acid receptor alpha (RAR alpha) gene with the novel gene PML, resulting in the expression of a PML/RAR-alpha fusion protein. Immunohistochemical examination of APL cells shows a unique abnormal distribution of anti-PML and anti-RAR alpha antibody labeling. The PML labeling pattern observed in normal cells consists of 5 to 10 discrete spherical nuclear bodies called PODs (for “PML oncogenic domains”), whereas that of APL consists of a smaller and far more numerous speckled pattern. We examined malignant cells from patients with a variety of hematopoietic cancers by immunohistochemistry (IH) and found this abnormal PML pattern expressed in cells from patients with t(15;17)-associated leukemia but not in patients with other neoplastic disorders. IH results agreed with reverse transcription polymerase chain reaction for PML/RAR-alpha in 31 of 32 patients with acute myelogenous leukemia, including 5 of 5 patients in whom the initial clinical diagnosis of APL was not supported by cytogenetics, molecular tests, or response to all-trans retinoic acid (RA). Cells from patients with APL were examined during the course of retinoid therapy and at the time of complete remission and relapse. Reorganization of the PML labeling into PODs with normal appearance was observed in cells from patients who received RA. IH showed primarily normal PML staining during clinical remission, although the APL-specific labeling pattern was again seen in cells taken from patients at the time of relapse. Thus, IH provides an independent assay for the presence and expression of the molecular rearrangement of APL. The relative ease and speed of detecting the APL- specific PML labeling pattern should make IH a useful diagnostic tool to guide specific therapy of APL, and establish a direct assay for PML/RAR-alpha protein expression and localization in individual patient cells.