This study will examine whether the drug pyrimethamine can shrink lymph nodes and spleen in patients with autoimmune lymphoproliferative syndrome (ALPS). In this disease, lymphocytes (white blood cells) do not die as they normally would. As a result, patients have enlarged lymph glands, spleen, or liver, and other problems that may involve blood cell counts and autoimmune disease (overactivity of the immune system). Pyrimethamine is an orally administered antibiotic that has been used to treat or prevent malaria and toxoplasma, and may be effective in shrinking lymph nodes and spleen.

Patients with ALPS who are between 2 and 70 years of age and have had lymph gland enlargement for at least 1 year may be eligible for this study. Candidates will be screened with a medical history and physical examination, blood tests, and possibly a bone marrow test. Females of reproductive age will be screened with a urine pregnancy test. Women who are capable of becoming pregnant must use an effective method of birth control during the entire study period, because, taken during early months of pregnancy, pyrimethamine can cause birth defects in the fetus. Women who are pregnant or nursing are excluded from the study.

Participants will undergo the following tests and procedures:

• CT scan: For this test, the patient lies still in the CT scanner while images are taken of the neck, chest, and stomach area. A contrast dye is injected into a vein to brighten the CT images. Very young children will be evaluated on a case by case basis to determine whether a CT scan will be performed.
• Bone marrow biopsy: Participants undergo this test to rule out underlying bone marrow disease if they have not had a bone marrow test done in the last six months prior to enrolling in pyrimethamine study, as pyrimethamine can affect bone marrow function. Under local anesthesia, a needle is inserted into the back part of the hipbone and a small amount of marrow is removed. (Children are sedated for this test.)
• Leukapheresis: This is a procedure for collecting a small proportion of circulating white blood cells while conserving the majority of blood cells. Specifically, blood is drawn from a needle placed in an arm vein and is directed into a cell separator machine, which separates the blood cells by spinning. A small proportion of circulating white cells are removed, and the red cells, platelets, plasma and majority of white cells are returned to the patient's blood circulation. Only patients who are 7 years of age or older and weigh at least 55 pounds undergo this procedure. Other participants who choose not to have apheresis will have about 3 tablespoons of blood drawn instead.
• Pyrimethamine administration: When the above tests are completed, participants begin taking pyrimethamine. The dose is determined according to the individual's weight and is gradually increased during the study period. Patients take the drug twice a week for a total of 12 weeks.
• Blood tests: Blood samples are collected during weeks 2, 4, 6, 8, and 10 after beginning treatment, and 2 weeks after the last dose of pyrimethamine. The purpose of these blood tests is to check for possible drug-related side effects. Patients who develop a skin rash, mouth sores or other side effects may have one or more doses of the treatment drug withheld. When indicated, the patient will be directed to stop taking the study drug. If needed, drug side effects will be treated with a vitamin supplement, folinic acid, taken by mouth, 3 times weekly.
• Evaluations at the NIH Clinical Center will comprise of a pretreatment visit, one end of treatment visit at the end of 12 weeks and an optional post-treatment visit 3months after stopping pyrimethamine therapy.

Patients who respond well to treatment may be asked to return to NIH for additional visits at 3, 6, and 12 months after the treatment has ended for repeat evaluations. If their lymph glands or spleen become much larger after stopping pyrimethamine, they will be offered treatment for another 12 weeks. If they respond to the second course of treatment, they will return to NIH again after 3, 6, and 12 months. If the symptoms return again, patients will be asked to resume treatment for an additional 6 months or more. They will have blood drawn periodically by their private physician and will return to NIH for evaluation every 12 weeks.