SummaryLong-term treatment with the combination trimethoprim-sulphamethoxazole nearly always causes hematologic disturbance. The most frequent changes in sixteen patients, treated for 6 weeks or more, were : hypersegmentation of the granulocytes, decrease of the reticulocytes, elevation of indirect bilirubine and a shift to the left in the myeloid series of the bone marrow. Three patients had a moderate and reversible granulopenia. These side effects illustrate the necessity of hematological controls in prolonged therapy by this antibiotics. This control is indicated even for short-term treatment in patients with possible disturbance of folic acid metabolism : in two cases, an alcoholic with malnutrition and an epileptic patient treated by barbiturates, the authors observed a granulocytopenia already after twelve days.