To determine whether autologous (auto) or allogeneic (allo) stem-cell transplantation (SCT) improves outcome in patients with transformed follicular lymphoma compared with rituximab-containing chemotherapy alone. This was a multicenter cohort study of patients with follicular lymphoma and subsequent biopsy-proven aggressive histology transformation. Patient, treatment, and outcome data were collected from each transplantation center and combined for analysis. A separate control group was composed of patients with transformation treated with rituximab-containing chemotherapy but not SCT. The primary end point was overall survival (OS) after transformation. One hundred seventy-two patients were identified: 22 (13%) treated with alloSCT, 97 (56%) with autoSCT, and 53 (31%) with rituximab-containing chemotherapy. Five-year OS after transformation was 46% for patients treated with alloSCT, 65% with autoSCT, and 61% with rituximab-containing chemotherapy (P = .24). Five-year progression-free survival (PFS) after transformation was 46% for those treated with alloSCT, 55% with autoSCT, and 40% with rituximab-containing chemotherapy (P = .12). In multivariate analysis, patients treated with autoSCT had improved OS compared with those who received rituximab-containing chemotherapy (hazard ratio HR, 0.13; 95% CI, 0.05 to 0.34; P < .001). On the other hand, there was no OS difference between those treated with alloSCT and rituximab-containing chemotherapy (HR, 0.44; 95% CI, 0.16 to 1.24; P = .12). OS and PFS after SCT were similar between those treated with autoSCT and alloSCT. Five-year transplantation-related mortality was 23% for those treated with alloSCT and 5% for autoSCT. Patients undergoing autoSCT had better outcomes than those treated with rituximab-containing chemotherapy alone. AlloSCT did not improve outcome compared with rituximab-containing chemotherapy and was associated with clinically significant toxicity.