Patients who relapse post-ABMT are usually resistant to conventional therapy, and a potentially curative therapy with allogeneic BMT is limited due to availability of a matched donor. To assess whether such patients can be salvaged using partially mismatched related donors (PMRD), eight patients age 6-50 years old underwent PMRD-BMT. All patients ALL (n = 3) and AML (n = 5) were in relapse 7-31 months after first BMT. Donors (1-3 Ag mismatch) were selected from family members. Conditioning included TBI, etoposide, Ara-C and cytoxan (n = 3), or busulfan, thiotepa, and etoposide (n = 5). GVHD prophylaxis consisted of partial T cell depletion followed by systemic immunosuppression. All evaluable patients established sustained engraftment by day 18. Severe regimen-related toxicity was evident in the gastrointestinal and hepatic systems (6/8 and 4/8, respectively), the latter associated with poor outcome (P < 0.014). Acute but not chronic GVHD, grade > or = II occurred in 3/7 patients. Four of eight patients are disease-free, maintaining longer remission than following their first BMT (14 vs 9 months). In conclusion, our data shows that PMRD-BMT is a feasible option for patients who relapse post-BMT and use of such alloreactive grafts may be appropriate earlier in the disease course of high risk patients.