The importance of smoking in the aetiology of polycythaemia has been assessed in a group of patients referred to a general haematology clinic. All patients with true and relative polycythaemia (excluding those with polycythaemia rubra vera) were studied. Of the 21 such patients evaluated, 14 were smokers and had raised carboxyhaemoglobin levels and had no other demonstrable cause for their polycythaemia. The commonest physiological abnormality in these patients was a raised red cell mass combined with a low plasma volume. Six of the 14 patients were able to reduce their smoking with subsequent improvement in their haematocrits. These results suggest that smoking is a major cause of polycythaemia in an unselected series of referrals to a general haematology clinic. The early identification of these patients may be useful in planning therapy.