We experienced two cases of concurrent cytomegalovirus (CMV) and Pneu-mocystitis carinii pneumonia (PCP) in a 38-year-old female with severe aplastic anemia (SAA) and in a 37-year-old male patient with myelodysplastic syndrome (MDS) following allogeneic bone marrow trasplantation (BMT). The pneumonia was complicated by severe hypoxia, requiring ventilator support. One patient recovered following treatment with trimethoprim-sulfamethoxazole (TMP-SMX), ganciclovir and intravenous immunoglobulin (IVIG) but, one patient did not recovered despite of TMP-SMX, ganciclovir, IVIG and prednisone treatment. Our cases demonstrate that concurrent PCP and CMV pneumonia may occur following BMT. Routine prophylaxis with TMP-SMX for PCP and preventive measures such as surveillance culture and secondary prophylaxis with ganciclovir for CMV may be warranted in patients whose cellular immune response is compromised following allogeneic BMT.