Sixteen patients presented with B cell leukaemia (white cell count 26-269 x 10(9)/l) which could not be classified as chronic lymphocytic (CLL), prolymphocytic leukaemia, or follicular lymphoma in leukaemic phase. Eleven patients (10 men, one woman) corresponded histologically to intermediate (INT) or mantle zone lymphoma, and five, with less well defined features, were designated small lymphocytic lymphoma with cleaved cells. The blood films showed a pleomorphic picture with lymphoid cells of predominantly medium size with nuclear irregularities and clefts. The membrane phenotype of the circulating cells showed strong immunoglobulin staining and reactivity with CD5 and FMC7 in all cases tested; CD10 was positive in six out of nine cases. The membrane phenotype of two of the five cases of small lymphocytic lymphoma was close to those of B-CLL and three resembled INT lymphoma. Bone marrow trephine biopsy specimens showed a diffuse pattern of infiltration in INT lymphoma. The median survival of these patients was less than two years, suggesting that a leukaemic presentation is associated with poor prognosis. By combining data from histology, membrane markers, and peripheral blood morphology, the leukaemic phase of typical INT lymphoma can be defined in most cases.