PURPOSE Prior observations suggest a higher risk of transformation of nodular lymphocyte-predominant Hodgkin's lymphoma (NLPHL) to aggressive lymphoma, most commonly diffuse large B-cell lymphoma (DLBCL), than in classical Hodgkin's lymphoma. We evaluated the frequency of transformation in all patients diagnosed with NLPHL at the British Columbia Cancer Agency with long-term follow-up. PATIENTS AND METHODS The Lymphoid Cancer Database of the British Columbia Cancer Agency was searched to identify all patients diagnosed with NLPHL between 1965 and 2006. After pathologic review, 95 patients with NLPHL were confirmed. Results Patients with NLPHL had the following characteristics at diagnosis: median age of 37 years, 73% male, and 68% stage I or II disease. With a median follow-up time for living patients of 6.5 years (range, 2.5 to 33 years), 13 patients (14%) experienced transformation to aggressive lymphoma (median time to transformation, 8.1 years; range, 0.35 to 20.3 years). The actuarial risk of transformation to aggressive lymphoma was 7% and 30% at 10 and 20 years, respectively. Transformation was more likely in patients with initial splenic involvement (P = .006) at the time of diagnosis of NLPHL. The 10-year progression-free and overall survival rates in patients with transformed lymphoma were 52% and 62%, respectively. CONCLUSION The risk of transformation in patients with NLPHL to DLBCL is substantial and underappreciated. Because transformation can occur years after the primary diagnosis of NLPHL, long-term follow-up of these individuals is necessary to accurately estimate the risk of development of secondary DLBCL.