High-dose chemotherapy and haematopoietic stem cell (HSC) transplantation is considered standard therapy in patients with chemosensitive relapsed diffuse large B cell lymphoma (DLBCL). BCNU (carmustine), etoposide, cytarabine and melphalan (BEAM) is a widely used standard DLBCL conditioning regimen. The practice of basing chemotherapy doses on body surface area (BSA) is empirical and the best biometric parameter to dose chemotherapy is unknown. Weight-based dosing has been suggested to better predict toxicity of the conditioning regimen. We correlated the dose/weight ratio with toxicity and overall outcome in a uniform cohort of 80 consecutive patients receiving HSC transplant for relapsed DLBCL at Mayo Clinic, Rochester, MN following BSA-dosed BEAM conditioning chemotherapy. Melphalan dose was used as surrogate for the entire regimen. Median age at the time of transplant was 62 (26-77) years; 65% were males. The median melphalan dose was 3.2 mg/kg (range 2.2-4.5). Patients who received >3.6 mg/kg of melphalan were more likely to have grade 3 or 4 mucositis (44.4% vs. 9.8%, P = 0.001) and prolonged hospitalization (median 13 vs. 7 d; P = 0.04). Dose/weight ratio did not correlate with cumulative incidence of relapse (P = 0.3) or survival (P = 0.8). Transplant physicians should consider limiting the dose of BEAM to the equivalent of 3.6 mg/kg of melphalan.