Extracorporeal Photopheresis (ECP) has been used as one of the treatments for graft versus host disease (GVHD). Responses were observed in skin, liver, GI tract, mouth, eye and lung. ECP does not cause immune suppression and therefore less risk of infection or relapse of malignancy. However, the exact mechanism of action of this expensive procedure is not fully understood and no predictors of response to ECP are known so far. This reflects the need for a better understanding of GVHD and its response to ECP. Blood samples from 15 patients who receive ECP as part of the treatment of GVHD at our institution will be collected. Samples will be analyzed using flow cytometer

Inclusion Criteria:

1. Patients who are recipients of allogeneic stem cell grafts.
2. Patients with the diagnosis of GVHD according to established criteria and physician decides to treat using ECP
3. Patient must have 100% chimerism with the donor
4. No recent donor lymphocyte infusion.
5. Patients must be able to sustain a platelet count and a hematocrit > 20,000/mL and > 27% respectively, with or without transfusions.
6. The absolute WBC must be >1500/mL
7. Patient must be willing to comply with all study procedures.

Exclusion criteria:

1. Patients who are unable to tolerate the volume shifts associated with ECP treatment due to the presence of any of the following conditions: uncompensated congestive heart failure, pulmonary edema, severe asthma or chronic obstructive pulmonary disease, hepatorenal syndrome.
2. Active bleeding.
3. International normalized ration (INR) >2.
4. Patients with known hypersensitivity or allergy to psoralen.
5. Patients with known hypersensitivity or allergy to both citrate and heparin.
6. Patients with co-existing photosensitive disease (e.g. porphyria, systemic lupus erythematosus, albinism) or coagulation disorders.
7. Active, uncontrolled infection

Patients who are recipients of allogeneic stem cell grafts with the diagnosis of GVHD according to established criteria and physician decides to treat using ECP