Three cases of monocytic leukaemia in dogs are reported. Clinically, they all showed dullness, lymph node enlargement and hepato-spenomegaly and the overt illness was of short duration. Haematologically, the white blood cell count was raised to different degrees in each case. The great majority of white cells were monocytes and monoblasts; their cytological, cytochemical and electron microscopic features are described. Only a mild degree of anaemia was present in all cases. Pathological examinations, carried out in two of the dogs, showed widespread leukaemic infiltration of bone marrow, spleen, lymph nodes, liver and other organs. Differentiation of this syndrome from other forms of haemopoietic neoplasia is discussed.