The frequency of chronic lymphocytic leukemia (CLL) varies a great deal from one population to another. Aspects of CLL in Maghreb and Europe were compared through 2 series of comparable importance, studied during the same period and under similar conditions in Algiers and Amiens. This comparison shows, in Algeria, a lower frequency, a very marked male predominance, a high proportion of patients with a normal or high rate of gammaglobulins and an almost complete lack of hypogammaglobulinema. Demographic (compared structure of populations), technical (medical density, availability of laboratory examinations), and ethnic (rarity of CLL in some populations) factors are to be taken into consideration in explaining these differences.