Unusually severe infections phenomena were observed in three patients with chronic lymphocytic leukemia (CLL) who had undergone allogeneic bone marrow transplantation (BMT) from matched sibling donors. The first developed three episodes of cytomegaloviremia requiring anti-viral therapy; the third episode accompanied by cytomegalovirus hepatitis which required prolonged therapy with foscarnet. Another had Listeria monocytogenes meningitis which was difficult to eradicate and required prolonged maintenance antimicrobial therapy with oral trimethoprim-sulfamethoxazole and intrathecal gentamicin until death due to chronic graft-versus-host disease. The third patient had cytomegaloviremia lasting 47 days, which did not clear within 4 weeks of full-dose ganciclovir. Although the number of patients is small, in our experience the problems encountered were unusually severe compared with patients allografted for other disease. We conclude that CLL patients undergoing allogeneic BMT may be at a higher risk of infectious complications than patients allografted for other diseases, and require careful monitoring.