Non-Hodgkin's lymphomas can be slow-growing ("low-grade") or rapidly growing ("high-grade") cancer. For most patients, the cause is unknown, but lymphomas can develop in people with a suppressed immune system, such as after organ transplantation.

The tumors are graded according to their level of malignancy (aggressiveness)-- low grade, intermediate grade or high grade. Burkitt's tumor is an example of a high grade lymphoma. Non-Hodgkin's tumors occur more frequently than Hodgkin's lymphoma.

Most often non-Hodgkin's lymphomas affect people older than 50. High-risk groups include organ transplant recipients and immunosuppressed people. The incidence is 3 in 10,000 people.