11 patients with plasma cell leukaemia (PCL) are reported. Diagnostic clinical, haematological, immunological, biochemical and electron microscopical (TEM) data were analysed and compared to the largest series of PCL cases reported in the literature. Special attention was paid to four facets of this disease: (a) the clinical picture at admission; (b) the frequency of PCL; (c) the production of M components in relation to the maturity and type of the asynchronous plasma cells, and (d) the diagnostic problems of this entity of acute leukaemia of the afferent limb of the B lymphocyte transformation. In this series PCL emerges as a distinct clinical entity: patients are severely anaemic, hepatosplenomegaly is prominent, bone lessions are uncommun, but if present are usually non-osteolytic, and the response to treatment with an alkylating agent and glucocorticoid is poor. The diagnosis is difficult since the circulating plasma cells may have morphological features which only allows the diagnosis to be made after the TEM examination. If the peripheral blood of cases of acute leukaemias and immunocytic dyscrasias is routinely examined by TEM, PCL appears to be a not uncommon variant of plasma cell dyscrasia -in the present study it was 11%.