Bone marrow transplantation has become an accepted mode of treatment for children with acute myelocytic leukemia in their first remission and acute lymphocytic leukemia after their first bone marrow relapse. Two-year survival rates of 50% can be achieved in patients undergoing transplant during remission, in contrast to a 2-year survival of 15% in those undergoing transplant while still in marrow relapse. Recurrence of bone marrow leukemia relapse is a significant cause of marrow transplant failure. Overt or occult testicular relapse occurs in 10–15% of males with acute lymphocytic leukemia receiving or having completed standard therapy regimens for control of their disease and frequently leads to a subsequent bone marrow relapse. This paper describes a child with acute lymphocytic leukemia who received a successful marrow transplant following bone marrow relapse and developed testicular leukemia relapse approximately 20 months after transplant. The experience with this child suggests that bilateral testicular biopsies should be a mandatory part of the routine evaluation to screen for residual leukemia before bone marrow transplantation.