This booklet provides information for patients and their families about the use of blood or marrow stem cell transplantation for the treatment of leukemia, lymphoma, or myeloma. A glossary at the end of the booklet may help the reader understand the technical terms. We hope this information is of assistance. Comments as to clarity of the information provided and the omission of additional information that would have been helpful are welcome. Between 1970 —— when the transplant registry began tabulating statistics —— and today, the frequency of stem cell transplantation for the treatment of hematological malignancies has increased from hundreds to thousands of patients transplanted per year. Each year 9,000 persons in North America undergo stem cell transplantation for leukemia, lymphoma, or myeloma. The procedure continues to be improved in anticipation of making it avail- able to even more patients each year. Before describing the technique and its application further, a brief description of normal blood and marrow is provided for background.