Diffuse large B-cell lymphomas can be considered primary bone tumors if they are monostotic or polyostotic, affecting multiple skeletal sites without visceral or lymph node involvement. They are rarely considered as extranodal lymphomas or as bone tumors, respectively. To elucidate the prognostic relevance of clinicopathologic characteristics in such disease, we collected a cohort of primary diffuse large B-cell lymphomas of the bone and retrospectively investigated 33 patients. The cohort encompassed the years 1975 to 2004. Protein expression patterns were identified by immunohistochemistry applied to a tissue microarray. The patients included 23 males (mean age, 37 years) and 10 females (mean age, 54 years). Disease stage was I and II in 30 and IV in 3 patients. Within the mean follow-up of 28 months, 6 patients died. Median overall survival was reached after 78 months. Clinical factors favoring a good prognosis were age younger than 53 and administration of chemotherapy. Of the phenotypic markers analyzed (CD10, CD44s, CD138, Bcl-2, Bcl-6, MUM1, and Ki-67), MUM1 expression in more than 10% of the tumor cells and CD10 expression in less than 55% as well as a nongerminal center signature substantiated adverse outcome in a univariate model. In summary, poor survival in PB-DLBCL was clearly predicted in patients older than 53, who had not received chemotherapy, and who demonstrated MUM1 expression and nongerminal center phenotype.