These diseases occur predominantly in organ transplant patients and HIV-infected patients, but may appear in patients with congenital immune defects or those having immunosuppressive therapy for other conditions. Some of these lymphoid proliferations are benign, some are malignant, and some are initially benign (using current tests) but later acquire clinical and laboratory evidence of malignancy. Immunosuppressive lymphoproliferative disorder (ILD) develops in about 2% of all organ transplant patients, about 1% (range, 0.6%-15%) of bone marrow transplants, 2% (1%-17%) of kidney transplants, 3% (1%-8.6%) of liver transplants, and about 10% (1.8%-20%) of heart transplants. The lymphocytes in ILD are usually B-cells and are frequently heterogeneous (various B-cell types). Spread to areas outside of lymph nodes is common. Evidence of Epstein-Barr virus infection is extremely common. Four percent to 29% of patients with human immunodeficiency virus (HIV) infection develop ILD, with a very high incidence of non–lymph node sites such as the CNS and GI tract.