Twenty-five patients with intermediate- or high-grade non-Hodgkin's lymphoma (NHL) were treated by allogeneic bone marrow transplantation. For the nine patients transplanted in first complete remission the disease-free survival was 100%; of the nine patients transplanted in subsequent remissions four (44%) achieved long-term disease-free survival but two died of relapsed lymphoma. For the seven patients transplanted at a time when they had active disease, the initial complete remission rate was high but four died of progressive lymphoma and no patient achieved long-term disease-free survival. The incidence of complications associated with allogeneic bone marrow transplantation was similar to that observed in other series of patients transplanted for haematological malignancy and was related to the status of the disease at the time of transplant. Thus allogeneic bone marrow transplantation is a radical but effective treatment for patients with NHL who have 'minimal residual disease'. Efforts to define further the subset of patients who can most successfully be treated by transplantation may improve the overall results.