Follicular lymphoma (FL) frequently transforms into diffuse large B-cell lymphoma (DLBCL). To clarify the associated clinicopathological prognostic parameters, we examined the correlation of 11 histopathological parameters with progression-free survival (PFS) and overall survival (OS) in 107 consecutive patients who had DLBCL with pre-existing (asynchronous) or synchronous FL. The patients comprised 58 men and 49 women with a median age of 56 years. For DLBCL, the complete response rate was 81%, overall response rate was 88%, and 5-year PFS and OS rates were 55% and 79%, respectively. Immunohistochemical analysis of the DLBCL component revealed the following positivity rates: CD10, 64%; Bcl-2, 83%; Bcl-6, 88%; MUM1, 42%; GCB, 82%; cMyc index ≥80%, 17%; and Ki-67 index ≥90%, 19%. IGH/BCL2 fusion was positive in 57% of DLBCL cases. In univariate analyses, asynchronous FL and DLBCL (24%, P = 0.021), 100% proportion of DLBCL (29%, P = 0.004), Bcl-2 positivity (P = 0.04), and high Ki-67 index (P = 0.003) were significantly correlated with shorter PFS. Asynchronous FL and DLBCL (P = 0.003), 100% proportion of DLBCL (P = 0.001), and high Ki-67 index (P = 0.004) were significantly correlated with shorter OS. In a multivariate analysis, asynchronous FL and DLBCL (P = 0.035) and 100% proportion of DLBCL (P = 0.016) were significantly correlated with shorter OS. Thus, asynchronism and 100% proportion of DLBCL, that is, FL relapsed as pure DLBCL, or FL and DLBCL at different sites, were significant predictors of unfavorable outcome of patients with DLBCL transformed from FL.