We experienced two cases of heparin-induced thrombocytopenia (HIT) which occurred during unfractionated heparin treatment. The first patient was a 72-year-old man, who was admitted to our hospital because of sudden onset dyspnea in January 2000. He was diagnosed as having a pulmonary embolism and heparin was started. Nine days later, progressive embolization of the pulmonary artery and femoral vein was found and thrombocytopenia (platelet count 20 x 10(9)/l) was observed 14 days after that. Cessation of heparin and administration of argatroban resulted in progressive normalization of the platelet count. The second patient was a 62-year-old woman, who was admitted to our hospital in April 2001, with the chief complaint of sudden onset dyspnea. She was diagnosed as having acute left-sided heart failure and heparin was started. Fifteen 15 days later, thrombocytopenia (platelet count 17 x 10(9)/l) was observed. Cessation of heparin resulted in normalization of the platelet count. Both cases were positive for anti-heparin-platelet factor 4 (PF4) antibody. Here we report on the clinical course of two cases of HIT with a review of the literature.