Six children (aged 9 months to 10 and 5/12 years) with hemophagocytic lymphohistiocytosis (HLH) have undergone allogeneic BMT in Sweden. In two of the children unrelated donors were used. Parents were available as donors in two of the cases and siblings in the other two. Conditioning before BMT consisted of etoposide, busulfan and cyclophosphamide with the addition of ATG in two cases and OKT 3 in one case. For post-transplant immunosuppression, i.v. methotrexate and cyclosporin A (CsA) were used in five cases, and in one child CsA was combined with methylprednisolone. Of the six children, four are alive and well 2 and 3/12 to 3 and 1/12 years after BMT. One child, who had an unrelated donor with one DR-antigen mismatch, died 30 days after BMT of fulminant grade IV GVHD. Another patients, seropositive for CMV, received marrow from an unrelated HLA-A, -B, -DR and -DP identical donor. After an initially uneventful course, CMV was isolated from her leukocytes. Seven months after BMT she developed a progressive obstructive chronic bronchiolitis and succumbed to respiratory insufficiency 14 months after the transplant. This study supports the view that BMT is the treatment of choice in HLH, particularly if an HLA-identical related donor is available.