Summary1. The author studied the age-distribution curves of the different forms of leukemia, on the basis of the National Statistics of Diseases and two series of clinical observations from the University of Ghent (Belgium).2. The acute leukemia is the most frequent form to day. This form is most seen at the age between 3 to 4 years and between it to 13 years. The most cases are actually seen past the fourth decade.3. Only the chronic lymphatic leukemia is more frequent on males. The differences between sexes is rather small, concerning the other two forms of leukemia.4. To-day the incidence of leukemia after the fourth decade seems to increase slowly. This phenomenon is not yet clear understood.