The histological features of prolymphocytic leukaemia are described in spleen, lymph nodes, liver and bone marrow from nine cases. Eight patients had B‐PLL one had T‐PLL. These features were compared with those of six cases of B chronic lymphocytic leukaemia (B‐CLL) with massive splenomegaly. Both PLL and B‐CLL showed enlargement of the white pulp with nodule formation and diffuse infiltration of the red pulp. In both PLL and CLL larger cells were found in the white pulp nodules often concentrated on the periphery producing a bizonal appearance. The extent of involvement of the white pulp was greater in PLL than in uncomplicated CLL. However, this was not so in two cases of B‐CLL in prolymphocytoid transformation in which the heavy replacement of the white pulp by large cells without a bizonal arrangement had effaced the red pulp. The cytological differences between PLL and CLL were best appreciated in splenic red pulp. The cells of CLL were small lymphocytes with clumped chromatin, those of PLL were larger with bigger nuclei, often indented in some cases, and distinct nucleoli. The pattern of infiltration in the bone marrow, liver and lymph nodes was similar in PLL and CLL. In conclusion, PLL can be distinguished from CLL by morphological and immunological features. The distinction is important clinically, because the survival of PLL is shorter than th