The amount and distribution of tissue mast cells in the three subtypes of immunocytoma (IC) were studied in lymph nodes of 58 cases and compared with the findings on 34 cases of chronic lymphocytic leukemia (CLL). There were significantly more mast cells in the lymphoplasmacytic and lymphoplasmacytoid subtypes of IC than in CLL. The median mast cell count for the polymorphic subtype of IC was also greater than that for CLL; however, this difference was not statistically significant. Tissue mast cells were diffusely distributed in the lymph nodes in IC, whereas they were chiefly located in the sinus in CLL. Moreover, the cells themselves and their granules were generally larger in IC. Increase in the number and altered distribution of the tissue mast cells in histological sections are therefore diagnostic aids for distinguishing IC from CLL.