Criteria for Inclusion:

• Males and females ages birth to 16 years,inclusive, at enrollment. For neonates, the minimum corrected gestational age should be 37 weeks.
• Documented HIT defined as a fall in platelet count to less than 100,000/uL or a >=50% decrease in platelets after the initiation of heparin therapy with no apparent explanation other than HIT, or
• Require anticoagulation and have a documented history of a positive HIT test in the absence of current thrombocytopenia (i.e. patients at risk for HIT), or
• In the opinion of the investigator, require alternative anticoagulation due to an underlying condition, such as AT-III deficiency or heparin resistance.
• Signed (written) informed consent or assent by the patient (if age appropriate) and the patient's parent or guardian.

Criteria for Exclusion:

• Any condition which, in the investigator's opinion, would contraindicate the use of Argatroban or would endanger the patient if he/she participated in this trial.
• Unexplained aPTT > 2 times the upper limit of normal at baseline, in the absence of heparin.
• International Normalized Ratio (INR) >1.6 at baseline in the absence of warfarin.
• Known clinical site of bleeding or predisposition to bleeding (e.g., GI bleed, hematuria, hemorrhagic CVA,retroperitoneal hematoma, diabetic retinopathy, hemorrhagic pericardial effusion, or hemorrhagic pleural effusion. Patients who have a known clinical site of bleeding may be enrolled if the investigator deems the risk of continued thrombosis to outweigh the potential bleeding risk.
• Any patient who has received any investigational medication within 30 days prior to the first dose of study medication or who is scheduled to receive an investigational drug other than Argatroban during the course of the study.
• Known hypersensitivity to Argatroban or chemically related compounds.
• Females of childbearing potential who are pregnant (positive serum beta-HCG), breast feeding, or sexually active and not taking adequate contraceptive precautions (e.g. IUD or oral contraceptives).
• Any patient receiving a thrombolytic medication (e.g. tPA).
• Any neonate with a corrected gestational age of < or = 44 weeks and bleeding in the head (Grade I or above) as determined by head ultrasound.