Abstract: Objective To analyze Fms-like tyrosine kinase 3 (FLT3) gene and FLT3 internal tandem duplication (ITD) mutation in acute lymphoblastic leukemia (ALL) patients of different immunological subtypes. Methods Polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP) was used to detect FLT3 gene and FLT3/ITD mutation in 63 ALL cases. Results Among the 63 ALL cases, FLT3 gene was detected in 41 (61.5%) cases. The positivity rate of FLT3 gene in pre-pre B-lineage ALL, pre-B-ALL, B-lineage ALL and T-lineage ALL cases were 93.3% (14/15), 77.8% (14/18), 41.7% (5/12) and 28.6% (4/14), respectively. The positivity rate of FLT3 gene was significantly higher in pre-pre B-ALL/pre B-ALL subtypes (84.8%) than in B-ALL subtypes (41.7%, P<0.005), and the rate was significantly higher in B- ALL subtypes (73.3%) than in T-ALL subtypes (28.6%, P<0.001). Two cases (3.2%) were found to have FLT3/ITD mutation, which were also positive for myeloid antigen expression and diagnosed as acute mixed-lineage leukemia, showing leukocytosis and high percentage of bone marrow blast cells with poor prognosis. Conclusions FLT3 gene can be detected in both B- and T-lineage ALL patients, but more frequently in the former. In B-lineage ALL patients, FLT3 gene is more frequent in cases with undifferentiated than those with differentiated blast cells. FLT3/ITD is rarely detected in ALL patients and FLT3/ITD mutation detection might be helpful to identify the genotypes and evaluate the prognosis of acute leukemia.