Two patients with indolent B‐cell lymphomas (small B‐cell and small cleaved cell) developed a coexistent malignant histiocytosis 3 and 6 years after diagnosis of their lymphomas. In both patients, malignant histiocytosis presented as new onset of fever, weight loss, and rapidly progressive bulky disease superimposed on a previously stable clinical course without constitutional symptoms. Both patients failed to respond to combination chemotherapy. The terminal aggressive phase of indolent B‐cell lymphomas usually represents transformation of the original neoplasm to a large non‐cleaved or immunoblastic B‐cell histology. Malignant histiocytosis has not previously been reported as a second neoplasm in patients with B‐cell lymphomas.