The leukaemias comprise a group of blood disorders which differ widely in regard to prognosis, the need for treatment and the intensity of treatment. A detailed understanding of them is necessary to decide which cases require treatment, and where the treatment is to be carried out. Such an understanding is also needed in counselling and advising patients, proper communication being a very important part of management. Acute leukaemia (with some exceptions) requires intensive and difficult treatment appropriate only in properly staffed hospital units. Chances of control range from reasonable to very good, especially in children, and significant prolongation of survival is seen in those who achieve remission. Future prospects for cure are promising. Chronic lymphatic leukaemia may be a very benign disease, though risk of infection increases as time passes. Chronic myeloid leukaemia, though initially benign, is less chronic than is generally believed and commonly the disease transforms to an acute pattern which cannot for long be controlled.