A 27-year-old pregnant woman was admitted to a local hospital because of headache, nausea, and general fatigue. Her blood examination showed leukocytosis, anemia, and thrombocytopenia. She was referred to our hospital in March 1998. Her bone marrow was normocellular with an excess of blasts (89.1%, peroxidase stain(-), PAS stain(-)) that displayed a positive immunophenotype for CD2, CD4, CD5, CD7, CD34, CD38, and CD71. Chromosome analysis revealed complex abnormal karyotypes. The patient was given a diagnosis of acute lymphoblastic leukemia associated with central nervous system and breast infiltration, and received induction chemotherapy during the second trimester of her pregnancy. After she achieved complete remission, a cesarean section was performed, and a healthy baby delivered. Our experience in this case demonstrated that combination chemotherapy during the second trimester of pregnancy is feasible.