We assessed the incidence, prognostic features, and outcomes associated with transformation of follicular lymphoma (FL) among 2652 evaluable patients prospectively enrolled in the National LymphoCare Study. At a median follow-up of 6.8 years, 379/2652 (14.3%) patients transformed following the initial FL diagnosis, including 147 pathologically confirmed and 232 clinically suspected cases. Eastern Cancer Oncology Group performance status >1, extranodal sites >1, elevated lactate dehydrogenase, and B symptoms at diagnosis were associated with transformation risk. Relative to observation, patients initiating treatment at diagnosis had a reduced risk of transformation (hazard ratio [HR], 0.58; 95% confidence interval [CI], 0.46-0.75). The risk of transformation was similar in patients treated with rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone compared with rituximab, cyclophosphamide, vincristine, and prednisone (adjusted HR, 0.94; 95% CI, 0.62-1.42). Maintenance rituximab was associated with reduced transformation risk (HR, 0.67; 95% CI, 0.46-0.97). Five-year survival from diagnosis was significantly worse for patients with vs without transformation (75%, 95% CI, 70-79 vs 85%, 95% CI, 83-86). The median overall survival posttransformation was 5 years. Forty-seven patients with evidence of transformation at the time of diagnosis shared similar prognostic factors and survival rates to those without transformation. Improved outcomes for transformation in the modern era are suggested by this observational study. This trial is registered at www.clinicaltrials.gov as #NCT00097565.