Aldehyde dehydrogenase (ALDH) 1 is expressed in various stem/progenitor cells, including cancer-initiating cells (CICs). In the present study, ALDH1 expression was immunohistochemically examined in diffuse large B-cell lymphoma (DLBCL). Unexpectedly, ALDH1 expression was occasionally detected in stromal cells, but not in lymphoma cells. Positively stained cells were CD68(+) cells with voluminous cytoplasm and fascin(+) cells with reticular morphology, designated as macrophages and dendritic cells (DCs), respectively. The presence of ALDH1(+) macrophages was not correlated with stage, response to therapies and prognosis. However, cases with ALDH1(+) DCs were mostly nodal, which showed a poorer response to therapies and a worse overall survival rate than cases without ALDH1(+) DCs. The presence of ALDH1(+) DCs appears to be involved in the malignant potential of DLBCL in lymph node.