The natural history and haematological features of 18 patients with a chronic form of myelomonocytic leukaemia are described. The majority were elderly and, in this series, females predominated. Haematological prodomata, such as unexplained monocytosis, leucopenia, or thrombocytopenia were common, and the clinical onset was insidious. Splenomegaly was variable but tended to increase as the disease progressed. Anaemia was usually less than in the acute disease, unless compounded by iron deficiency. The blood film typically showed a mixed monocytosis and granulocytosis, cells in both lines showing abnormalities. 'Paramyeloid' cells, appearing in Romanowsky stained films intermediate between myelocytes and monocytes, were characteristic, although cytochemical and electron microscopical analysis suggests that these cells may be allotted to one or other cell line. The marrow aspirate was characteristically hypercellular, showed granulocytic hyperplasia, and, in contrast to the well-differentiated blood picture, the proportion of poorly differentiated cells, including blasts, was high. Serum lysozyme levels were usually raised. Five of the 18 cases survived more than 5 years, while 10 lived 2 years or longer. The morphological and clinical features form part of a spectrum including acute myelomonocytic leukaemia, into which several of the patients transformed. Recognition of the syndrome is important because the patients are probably best managed without intensive chemotherapy.