Purpose: To determine the incidence and risk factors for the development of new malignancies occurring after stem-cell transplantation (SCT). Patients: Between January 1, 1974, and March 31, 2001, 3,372 patients underwent SCT at the University of Minnesota. From these transplants, 147 posttransplant malignancies (PTMs) were identified in 137 patients. Results: Excluding nonmelanoma skin cancers (n = 19) and carcinoma-in-situ (n = 5), the remaining 123 cases represented an 8.1-fold (95% confidence interval [CI], 6.7 to 9.6) increased risk of a PTM, an excess risk of 102.7 cases/ 10,000 persons/yr (age and sex adjusted). This includes a significantly elevated risk for developing myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML; standardized incidence ratio [SIR] = 300; 95% CI, 210 to 406), non-Hodgkin's lymphoma including posttransplant lymphoproliferative disorder (PTLD; SIR = 54.3; 95% CI, 39.5 to 41.1), Hodgkin's disease (SIR = 14.8; 95% CI, 3.9 to 32.9), or solid tumors overall (SIR = 2.8; CI, 2.0 to 3.7) and in specific for melanoma, brain, and oral cavity tumors. The cumulative incidence for the development of any PTM was 6.9% (95% CI, 5.2 to 8.6) at 20 years post-SCT. For PTLD (n = 43), the cumulative incidence plateaued at 1.4% (95% CI, 1.0 to 1.8) by 10 years post-SCT. For MDS or AML, the cumulative incidence plateaued at 1.4% (95% CI, 0.9 to 1.9) by 10 years post-SCT. The cumulative incidence of developing a solid tumor did not plateau and was 3.8% (95% CI, 2.2 to 5.4) at 20 years post-SCT. Conclusion: These data reveal that the risk of PTMs, especially solid tumors, continues to increase even 20 years after transplant, necessitating long-term close follow-up for these patients.