In patients with malignant blood disease, infection is the most serious complication. The prompt differential diagnosis of tuberculosis leads to an appropriate treatment. A method based on DNA amplification and hybridization for the rapid detection of Mycobacterium tuberculosis was used to test 10 clinical specimens (sputum, gastric aspirate and pleural effusion) from blood disease patients in whom tuberculosis was suspected. Mycobacterium tuberculosis DNA was detected in 3 specimens, including one which was negative on standard microbiological criteria (microscopy and/or culture). The other 7 specimens with fever or abnormal shadow on a chest X-ray were negative by both our method and the Standard microbiological criteria. Rapid diagnosis of tuberculosis by amplification of mycobacterial DNA in cases of blood disease in clinically useful.