The overexpression of the multidrug resistance gene MDR1 has been found to be associated with therapy-resistance in hematological malignancies. Yet the cellular mechanisms underlying this increased expression are completely unknown. Point mutations in the MDR1 promoter have been found in osteogenic sarcoma (Stein et al., Eur J of Cancer, 30A: 1541-1545, 1994). We therefore analyzed DNA from hematological malignancies for MDR1 promoter point mutations. Two pairs of overlapping PCR primers were designed which did not amplify the MDR3 gene. Amplified DNA was screened using single strand conformation polymorphism (SSCP). 139 patients and 93 normal controls were studied. Fifteen patients (11%) were found to have abnormal bands on the SSCP analysis. Of these, 9 had acute myeloid leukemia (AML), 4 chronic lymphocytic leukemia (CLL), 1 acute lymphocytic leukemia (ALL), and 1 nonHodgkin's lymphoma (NHL). Sequence analysis revealed that all patients were heterozygous for a point mutation in the promoter (T-C transition at +8). Four normals (4%) were found to be heterozygous for the mutation. Confirmation of the mutation was performed by oligonucleotide probe hybridization. All but two of the AML patients have died due to chemoresistant disease (one is lost to followup). Of the CLL patients, one is alive with progressive disease, and the others have died. Further studies will assess the effect of this mutation on MDR1 gene transcription.